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2020 Canadian Team Mathematics Contest
Solutions

May 2020

© 2020 University of Waterloo

Individual Problems

  1. Solution 1
    The numerator is equal to 24+12=36. The denominator is equal to 424=164=12, so 24+12424=3612=3

    Solution 2
    By factoring 4 out of both the numerator and denominator, we get 24+12424=4(6+3)4(41)=6+341=93=3.

    Answer: 3

  2. Solution 1
    Solving for k, we have k=103, so 65k2=65×1032=60152=42=2. Solution 2
    By factoring and substituting, we get 65k2=25(3k)2=25(10)2=42=2.

    Answer: 2

  3. Solution 1
    From the information given, E is the reflection in the y-axis of A and D is the reflection in the y-axis of B. Since the coordinates of A are (4,6), this means the coordinates of E are (4,6). Similarly, since the coordinates of B are (2,0), the coordinates of D are (2,0).

    The slope of segment DC is 0420=2. The slope of ED is 6042=3, so the sum of the slopes is 2+3=1.

    Solution 2
    The slope of segment AB is 062(4)=3. Since ED is the reflection of AB in the y-axis, its slope is the negative of the slope of AB. This means the slope of ED is 3. Similarly, the slope of segment BC is 400(2)=2, so the slope of DC is 2. Therefore, the sum of the slopes is 2+3=1.

    Answer: 1

  4. The odd numbers on the spinner are 1, 3, and 5. The wedges labelled by 3 and 5 each take up 14 of the spinner and so each will be spun with a probability of 14. If we let the probability of spinning 1 be x, then we have that the probability of spinning 2 is 2x. The probability of spinning either 1 or 2 is 14, which means x+2x=14 or 3x=14 so x=112.

    Therefore, the probability of spinning an odd number is 14+14+112=712.

    Answer: 712

  5. Suppose y=mx+b and y=nx+c are the equations two lines. If the two lines have the same slope, then the only way they can intersect is if they have they are the same line. To see this, suppose m=n and there is some x such that mx+b=nx+c. Since m=n, this equation is the same as b=c, which means the lines only intersect if b=c, but this would make the lines identical.

    On the other hand, if mn, then the two lines will intersect in exactly one point. To see this, suppose mn and mx+b=nx+c. Then (mn)x=cd, and since mn0, we have x=cdmn.

    The six lines that Maggie graphs can be broken into two sets of three distinct but parallel lines. Each of the three lines with slope 1 intersects each of the three lines with slope 2 exactly once by the previous paragraph. This gives a total of 3×3=9 intersections.

    We must be sure that none of the points have been counted more than once. For this to have happened, we would need to have at least three of the six lines intersecting in the same point. There are only two possible slopes, so among any three of the lines, at least two must have the same slope. Therefore, if three of the lines intersect in the same point, then two distinct lines of equal slope intersect. From the previous paragraph, this cannot happen. Therefore, the nine points are distinct, so n=9.

    Answer: 9

  6. The prime factorization of 60 is 22×3×5, so the prime factorization of 605 is (22×3×5)5=2103555. Therefore, the divisors of 605 are numbers of the form 2a3b5c where a, b, and c are integers with 0a10, 0b5, and 0c5.

    In order for a divisor to be a perfect square, each of a, b, and c must be even.

    Therefore, the divisors of 605 which are perfect squares are the numbers of the form 2a3b5c where a is equal to 0, 2, 4, 6, 8, or 10, b is equal to 0, 2, or 4, and c is equal to 0, 2, or 4. This gives a total of 6×3×3=54 ways of choosing a, b, and c so that 2a3b5c is a perfect square. This includes the divisor with a=b=c=0 corresponding to the divisor 1 which is a perfect square, so the answer is 541=53.

    Answer: 53

  7. Since 273=3, the dimensions of the larger cube must be 3×3×3.

    Therefore, each side of the cube has area 3×3=9, so the total surface of the cube is made up of 32×6=54 1 by 1 squares. Each of these unit squares is a face of one of the unit cubes. Since 13 of the surface area is red, this means 543=18 of these unit squares must be red.

    For any unit cube, there are either 0, 1, 2, or 3 of its faces showing on the surface of the larger cube. The unit cube at the centre of the larger cube has none of its faces showing, the 6 unit cubes in the centres of the outer faces have exactly one face showing, the 12 unit cubes on the edge but not at a corner have two faces showing, one of each of two adjacent sides, and the 8 unit cubes at the corners each have three faces showing.

    This means that at most three faces of any unit cube are on the surface of the larger cube, so we cannot possibly have 18 of the unit squares being coloured red if there are fewer than 183=6 red unit cubes. There are 8 unit cubes on the corners, so if we colour exactly 6 unit cubes red and the other 21 black, then arrange the cubes into a 3×3×3 cube so that the 6 red unit cubes are at the corners, there will be exactly 18 of the unit squares on the surface coloured red. Therefore, the answer is 6.

    Answer: 6

  8. Since Gina’s rate is 7 minutes per kilometre, she runs 17 kilometres every minute. Since there are 60 seconds in a minute, 15 seconds corresponds to 0.25 minutes. As long as t>0.25 minutes, the distance Gina has travelled at time t is equal to 17(t0.25)=t0.257 For t0.25 minutes, the distance travelled is 0.

    The rate in minutes per kilometre that the watch will display at time t is equal to total number of minutestotal number of kilometres=tt0.257=7tt0.25 Note that this formula only works for t>0.25. We will now use this to find the times at which the watch displayed 7 minutes 30 seconds per kilometre and 7 minutes 5 seconds per kilometre.

    First, 7 minutes 30 seconds per kilometre is the same as 152 minutes per kilometre, so the value of t satisfying 152=7tt0.25 is the time at which this is the rate displayed by the watch. This equation can be rearranged to 15(t14)=14t or t=154 minutes.

    Next, 7 minutes 5 seconds is 7560=7112=8512 minutes. To find the corresponding time, we set 8512=7tt0.25 and rearrange to get 85(t14)=84t so t=854.

    This means that Gina was running for 854154=704 minutes from the time when the app said 7 minutes 30 seconds per kilometre to the time when it said 7 minutes 5 seconds per kilometre.

    While in motion, she runs at a rate of 7 minutes per kilometre. This means she runs 7047=104=2.5kilometres in this time period.

    Answer: 2.5 km

  9. Connect O to each of A, C, D, and E as shown.

    Each of OA, OC, OD, and OE are radii of the circle and so are equal. As well, AC=CD=DE=EA since they are the side-lengths of a square, which means AOC, COD, DOE, and EOA are all congruent. This means AOC=COD=DOE=EOA and since the sum of these angles must be 360, AOC=COD=DOE=EOA=3604=90 By the Pythagorean theorem, OE2+OD2=ED2. Since the radius of the circle is 2, we have 22+22=ED2 or 2+2=4=ED2, so ED=2 since ED>0.

    Now draw a line from B through O to meet ED at H and AC at J.

    We know that EOD=90, which means that EOB+DOB=36090=270. Furthermore, OE, OD, and OB are all radii of the circle and hence are equal and BE=BD by construction, so EOB and DOB are congruent. This means EOB=DOB and since their sum is 270, we must have EOB=2702=135.

    Since BOH is a straight line, this means EOH=180135=45. We previously established that EOD is right and isosceles, which means OED=45 as well. This means OHE=1804545=90.

    We now have that EBH and DBH are right triangles having a common hypotenuse length and a shared side, which means they are congruent. Therefore, EH=HD. It follows that EH=1 because ED=2.

    Line segments AC and DE are parallel because they are opposite sides of a square. Therefore, BFJ=BEH and BJF=BHE. It follows that BFJBEH.

    Since BHE=90, we also have that AEBH which means AEF=JBF. Angles AFE and JFB are alternating and hence equal, and both EAF and BJF are right, which means EFABFJ. By similar reasoning, DGC is also similar to BFJ, BEH, and EFA.

    Since DGCEFA and AE=BC, we have that DGCEFA, which means AF=GC.

    Since EFABEH, we get that AFAE=HEHB=HEHO+OB=11+2 Since AE=2, this means AF=21+2=21+2×1212=222

    Since AF=GC, we have that GC=222 as well, and using that AC=2, we have FG=ACAFGC=2(222)(222)=642

    Since ACED, we have that BFGBED. This means their heights are in the same ratio as their sides. Therefore, BJBH=FGED and since BH=1+2, FG=642, and ED=2, this means BJ=BHFGED=(1+2)6422=(1+2)(322)=1+2 Therefore, the area of BFG is 12(FG)(BJ)=12(642)(1+2)=(322)(1+2)=7+52. The answer is (7,5).

    Answer: (7,5)

  10. By adding 2, 8, and 24 to both sides of the first, second, and third equations, respectively, we get ab+2ab2=56bc+4b+2c+8=308cd6c+4d24=77 The left sides of these equations can now be factored. The integers on the right sides of the equations have been factored into products of primes as well. (a1)(b+2)=23×7(b+2)(c+4)=22×7×11(c+4)(d6)=7×11 We are given that c and d are positive integers which means c+4 is a positive integer and d6 is an integer. This means c+4 is a positive divisor of 77, so we must have that c+4 is one of 1, 7, 11, and 77. We cannot have c+4=1 since this would mean c=3 which is negative.

    If c+4=7, then b+2=22×7×117=22×11 but this would mean a1=23×722×11=2×711 which is not an integer, but a1 must be an integer because a is an integer. Therefore, we cannot have c+4=7.

    If c+4=11, then b+2=22×7×1111=22×7, and so a1=23×722×7=2. Also, d6=7×1111=7, so we have (a1,b+2,c+4,d6)=(2,28,11,7) which gives (a,b,c,d)=(3,26,7,13) If c+4=77, then d6=1, b+2=22×7×1177=4, and a1=23×74=14. Therefore, (a1,b+2,c+4,d6)=(14,4,77,1) which gives (15,2,73,7)

    Answer: (3,26,7,13), (15,2,73,7)

Team Problems

  1. Since 11<16<19, we have 11<16<19 which is the same as 11<4<19.

    Answer: 4

  2. By factoring 33 out of the numerator, we have 353433=33(323)33=323=93=6

    Answer: 6

  3. Let the measure of the smallest interior angle be x. Then the other three interior angles measure 2x, 3x, and 4x.
    The interior angles of a quadrilateral sum to 360, which means 360=x+2x+3x+4x=10x Therefore, 360=10x, so x=36.

    Answer: 36

  4. The numbers being summed are the multiples of 101 from 10×101 through 20×101. Thus, if we set S to be the sum in question, we can factor out 101 to get S=101(10+11+12+13+14+15+16+17+18+19+20) It can easily be checked by hand that 10+11++19+20=165. Alternatively, using the fact that for any positive integer n we have 1+2+3+4++(n1)+n=n(n+1)2 the sum can be computed as 10+11++19+20=(1+2+3++19+20)(1+2++8+9)=20(21)29(10)2=10×219×5=21045=165 Therefore, S=101×165=(100+1)×165=16500+165=16665.

    Answer: 16665

  5. The number 10004444 is an eight-digit number having exactly four digits which are 4. We will show that this is the number we seek by showing that any other such number is larger than 10004444.
    Suppose we have an eight-digit number having exactly four digits which are 4. If a digit of 4 occurs as any of the first (leftmost) four digits, the number will be bigger than 10004444 (note that 1 is the smallest that the first digit can be since the number has eight digits).
    Otherwise, the four digits that are 4 occur as the last four digits. The smallest eight digit number ending in 4444 is 10004444.

    Answer: 10004444

  6. There were 200 visitors on Saturday, so there were 100 visitors the day before. Since tickets cost $9 on Fridays, the total money collected on Friday was $900.

    Therefore, the amount of money collected from ticket sales on the Saturday was 43($900)=$1200.

    Since there were 200 visitors on Saturday, the price of tickets on Saturday on that particular Saturday was $1200200=$6. Therefore, the value of k is 6.

    In fact, if all conditions are kept the same in this problem but the number of ticket sales on Saturday changes, the answer will still be 6. You might like to try to work out why this is true.

    Answer: 6

  7. An integer is divisible by both 5 and 9 exactly when it is divisible by 45. Since we seek a number having only even digits, its last digit is one of 0,2,4,6, or 8, so the number itself is even. Thus, we are looking for an even multiple of 45. An even number is a multiple of 45 exactly when it is a multiple of 90.
    Therefore, we are looking for the smallest 4-digit multiple of 90 that has only even digits.
    The smallest 4-digit multiple of 90 is 1080, but the first digit of this number is 1, which is odd.
    Each of the next 10 multiples of 90 has a first digit equal to 1, so the number we seek must be larger than 2000. The multiples of 90 that are at least 2000 are 2070,2160,2250,2340,2430,2520,2610,2700,2790,2880,2970 and the only number in this list that has all even digits is 2880. Therefore, 2880 is the smallest 4-digit number that is a multiple of 5, a multiple of 9, and has only even digits.

    Answer: 2880

  8. The larger semicircle has diameter 64 units. The length of a semicircular arc is πd2 where d is the diameter of the circle. This means the length of the large semicircular arc is πd2=π642=32π The diameter of each small semicircular arc is 644=16 units, so each small semicircular arc has length π162=8π There are four of these small arcs, so the perimeter of the figure is 32π+4(8π)=64π

    Answer: 64π

  9. By factoring and using exponent rules, we have 2010=(2×10)10=210×1010. Therefore, 2010=1024×1010, which is the integer 1024 followed by ten zeros. Thus, 2010 has eleven digits that are 0.

    Answer: 11

  10. Using exponent laws, we have 16x+182y1=24(x+1)23(2y1)=2(4x+4)(6y3)=22(3y2x)+7 Using the given condition that 3y2x=4, we get 16x+182y1=22(3y2x)+7=22(4)+7=21=12

    Answer: 12

  11. The sum of the areas of the triangles is equal to the area of AQ1Q7. This is because the triangles share vertex A, the base of AQ2Q3 begins where the base of AQ1Q2 ends, the base of AQ3Q4 begins where the base of AQ2Q3 ends, and so on with all of Q1, Q2, Q3, Q4, Q5, Q6, and Q7 lie on the x-axis.
    The height of AQ1Qy is the y-coordinate of A, which is 2. The first seven primes are 2,3,5,7,11,13,17, so p1=2 and p7=17. The base of AQ1Q7 has length p7p1=172=15. Therefore, the sum of the areas of the triangles is 12×15×2=15

    Answer: 15

  12. Since 90 is a multiple of 3, there are 903=30 multiples of 3 in the list. Since 90 is a multiple of 5, there are 905=18 multiples of 5 in the list. A multiple of 5 is a multiple of 3 exactly when it is a multiple of 15, which means the multiples of 15 were already crossed out when the multiples of 3 were crossed out. There are 9015=6 multiples of 15 in the list, so this means there are 186=12 multiples of 5 remaining in the list once the multiples of 3 have been crossed out. Therefore, there are 903012=48 numbers in the list once all of the multiples of 3 and 5 have been crossed out.

    Answer: 48

  13. By completing the square, we have y=14(x10)2+4, which means the vertex of the parabola is at A(10,4).
    If we set y=0, we have 14(x10)2+4=0 or 14(x10)2=4, so (x10)2=16. Taking square roots of both sides, we have x10=±4 or x=10±4. Therefore, the parabola passes through (6,0) and (14,0). From the information given, this means the coordinates of B are (6,0) and the coordinates of F are (14,0).
    The second parabola has its vertex at B(6,0), so its equation is y=a(x6)2 for some real number a. Using the fact that it passes through A(10,4), we get 4=a(106)2=16a, or a=14.
    Therefore, the equation of the second parabola is y=14(x6)2. Substituting x=0, we get y=14(6)2=364=9. Therefore, the second parabola crosses the y-axis at D(0,9).

    Answer: (0,9)

  14. Since the total mass of the first three fish was 1.5 kg, the average mass of the first three fish was 0.5 kg. Let M be the total mass of all of the fish. Since the average mass or the first three fish was the same as the average mass of of all of the fish,. This means M21=0.5 kg or M=10.5 kg. We can find the largest possible mass of any individual fish by determining the smallest possible mass of any twenty of the fish, then subtracting this mass from 10.5 kg.

    We first note that the answer is not 0.5 kg. This is because it is possible, for example, that 19 of the fish (including the first three) weighed 0.5 kg each and the other two weighed 0.2 kg and 0.8 kg. This distribution of weights satisfies all of the conditions.

    The largest fish Jeff could have caught would therefore be if 17 of the 18 fish caught after the first three weighed the minimum of 0.2 kg. This would mean the first three in addition to these 17 fish weigh 1.5 kg+17(0.2) kg=4.9 kg. The remaining fish would weigh 10.5 kg4.9 kg=5.6 kg.

  15. We will assume abcd. That is, that d is the largest of the integers. First, observe that if c6, then dc6, which means a2+b2+c2+d2>c2+d262+62=72>70. We are assume a2+b2+c2+d2=70, so this means we cannot have c6. It follows that a,b, and c are all at most 5.
    We will work through the possible values of d for which a,b,c, and d can be chosen with abcd and a2+b2+c2+d2=70.
    Since 92=81>70, we have that d8.
    If d=8, then a2+b2+c2=7082=6. The only perfect squares that do not exceed 6 are 12=1 and 22=4. It can easily be checked that 12+12+22=6 is the only way to express 6 as a sum of three positive perfect squares. Thus, we could have (a,b,c,d)=(1,1,2,8) and this is the only possibility with d=8.
    If d=7, we have a2+b2+c2=7072=21. The squares less than or equal to 21 are 12=1, 22=4, 32=9, and 42=16. The only way to write 21 as a sum of numbers in this list is 12+22+42. Thus, (a,b,c,d)=(1,2,4,7) is a the only possibility with d=7.
    If d=6, we have a2+b2+c2=7062=34. If c=5, then a2+b2=3452=9, and it can be checked that 9 cannot be expressed as a sum of positive perfect squares. Therefore, c5, and since c5, this means c4. If c<4, then c3, and since abc, we have a2+b2+c232+32+32=27<34. This means we must have c=4, so a2+b2=3442=18. The only way to write 18 as a sum of positive squares is 18=32+32, so this means (a,b,c,d)=(3,3,4,6) is the only possibility with d=6.
    If d=5, then a2+b2+c2=7052=45. If c=5, then a2+b2=4552=20. It can be checked that 22+42=20 is the only way to write 20 as a sum of two positive perfect squares. This means (a,b,c,d)=(2,4,5,5) is a possibility, and it is the only possibility with c=d=5.
    If c=4, then a2+b2=4516=29, and it can be checked that 29 cannot be written as the sum of two positive perfect squares, so c4. Thus, c3, so a2+b2+c23(32)=27<45. Therefore, (a,b,c,d)=(2,4,5,5) is the only possibility with d=5.
    Finally, if d4, then a,b,c, and d are each less than or equal to 4. This would mean a2+b2+c2+d24(42)=64<70, so we cannot have d4.
    Therefore, (a,b,c,d) must be one of (1,1,2,8),(1,2,4,7),(3,3,4,6),(2,4,5,5) and the sums of the entries in these quadruples are 12, 14, 16, and 16, respectively. Therefore, the largest possible sum is a+b+c+d=16 (and it is achievable in two different ways).

    Answer: 16

  16. Let C be the common centre of the two circles, R be the radius of the larger circle, and r be the radius of the smaller circle. Since AB is tangent to the smaller circle, we have that ACAB. This means ABC is right-angled at A. By the Pythagorean theorem and since AB=5, we then have AC2+AB2=BC2 or r2+52=R2. The area of the shaded region is equal to the area of the larger circle minus the area of the smaller circle, or πR2πr2=π(R2r2). Rearranging r2+25=R2, we have R2r2=25, so the area of the shaded region is π(R2r2)=25π.

    Answer: 25π

  17. A quadratic function of the form f(x)=ax2+bx+c with a>0 attains its minimum at x=b2a. If a<0, the function attains its maximum at b2a.
    For f(x)=x22mx+8m+4, the minimum occurs at x=2m2=m. The minimum value of f(x) is therefore f(m)=m22m(m)+8m+4=m2+8m+4. This is a quadratic function in the variable m. The leading coefficient is negative, so the maximum occurs at m=82(1)=4. Therefore, the maximum of these minimum values is 42+8(4)+4=20.

    Answer: 20

  18. We will require the following fact. If a and r are real numbers with 1<r<1, then the sum of the geometric series a+ar+ar2+ar3+ar4+ is a1r.
    We will compute this sum by finding the sum t1+t3+t5+ and t2+t4+t6+ and adding the results. For the first sum, we have 17+173+175+ which can be rewritten as 17+17172+17(172)2+17(172)3+. This is a geometric series with a=17 and r=149. We also have that 1<149<1, so the sum of the series above is 171149=497491=748. We now compute t2+t4+t6+. Similar to before, we can write the sum as 272+272172+272(172)2+272(172)+ which is a geometric series with a=272 and r=172. Thus, its sum is 2491149=2491=248. The sum t1+t2+t3+ is therefore 748+248=948=316.

    Answer: 316

  19. We need to choose 2 points out of 24, and there are (242)=24(23)2=12×23=276 ways to do this. Two points will form a triangle together with point A unless all three points lie on a common line. Therefore, we can get the answer by counting the number of ways to choose two points so that they and A all lie on the same line and subtract this number from 276.
    We now draw every line through A and some other point in the diagram. Note that many of these lines will be the same. For example, drawing a line through A and any of the other four points in the bottom row gives the same line regardless of which of the four points is chosen. There are 13 lines in total:

    Each line originates from A, and every point in the 5 by 5 grid intersects with exactly one line.

    Eight of these lines pass through only one of the points other than A, so there cannot be a pair of points different from A on such a line that both lie on a line through A. Eliminating these lines from the diagram, we have

    Now there are five lines left. Three lines cross five points (each), including A. The other two lines cross three points (each), including A.

    There are five lines remaining. Two of them contain exactly two points other than A, so each of these identifies one pair of points that, together with A, do not form a triangle.
    Each of the other three remaining lines contains four points other than A, so each of these three lines identifies (42)=6 pairs of points that, together with A, do not form a triangle. Therefore, there are 2762(1)3(6)=256 ways to choose two points from the array that, together with A, form a triangle.

    Answer: 256

  20. We will imagine that the track has been broken into 12 equal units. With this convention, Isabelle will begin to run once Lyla has travelled 4 units, and Isabelle will always run 5 units in the amount of time it takes for Lyla to run 4 units.
    When Lyla has run a total of 4 units, Isabelle has run 0 units. At this time, Isabelle starts to run, so when Lyla has run 4+4=8 units, Isabelle will have run 5 units.
    When Lyla has run 8+4=12 units, Isabelle will have run 5+5=10 units.
    When Lyla has run 12+4=16 units, Isabelle will have run 10+5=15 units, and when Lyla has run 16+4=20 units, Isabelle will have run 15+5=20 units. At this point, Isabelle passes Lyla for the first time.
    Since Isabelle runs faster than Lyla, Isabelle will always have run farther than Lyla from this point on. The points at which Isabelle passes Lyla will occur when Isabelle has travelled an integer number of laps, or a multiple of 12 units farther than Lyla.
    This means the 5th times Isabelle passes Lyla will occur when Isabelle has gained an additional 4×12=48 units on Lyla.
    Isabelle gains one unit on Lyla for every 4 units Lyla runs, which means that Isabelle will gain 48 units on Lyla when Lyla runs 4×48=192 units.
    Therefore, when Isabelle passes Lyla for the 5th time, Lyla has run a total of 192+20=212 units. There are 12 units in a lap, so this means Lyla will have run 21212=17812 laps.
    When Isabelle passes Lyla for the 5th time, Lyla will have completed 17 laps.

    Answer: 17

  21. Using a log rule, we have that logx2=2logx, so f(x)=2sin2(log(x))+cos(2log(x))5. Using the trigonometric identity cos2x=cos2xsin2x, we get f(x)=2sin2(log(x))+cos2(log(x))sin2(log(x))5 which simplifies to f(x)=sin2(log(x))+cos2(log(x))5 and finally using the identity sin2u+cos2u=1 for any real number u, we have f(x)=15=4 and this holds for all x>0. Therefore, f(π)=4.

    Answer: 4

  22. An important observation for this problem is that every integer is either a multiple of 3, one more than a multiple of 3, or two more than a multiple of 3. Suppose x+y+z is a multiple of 3. If x is a multiple of 3, then y+z must also be a multiple of 3. There are three ways this can happen: both y and z are multiples of 3, y is one more than a multiple of 3 and z is two more than a multiple of 3, or y is two more than a multiple of 3 and z is one more than a multiple of 3. If x is one more than a multiple of 3, then y+z must be two more than a multiple of 3. Again, there are three ways that this can happen. We summarize the possible remainders after x, y, and z are divided by 3 (either 0, 1, or 2) that will lead to x+y+z being a multiple of 3:

    x y z
    0 0 0
    0 1 2
    0 2 1
    1 0 2
    1 1 1
    1 2 0
    2 0 1
    2 1 0
    2 2 2

    Of the numbers from 1 to 10 inclusive, 3, 6, and 9 are multiple of 3 (a total of three), 1, 4, 7, and 10 are one more than a multiple of 3 (a total of 4), and 2, 5, and 8 are two more than a multiple of 3 (a total of 3). Thus, for example, there are 3×3×3=27 triples (x,y,z) so that x+y+z is a multiple of 3 and each of x, y, and z is a multiple of 3. Computing the total for each row in the table above, we have

    x y z
    0 0 0 3×3×3 =27
    0 1 2 3×4×3 =36
    0 2 1 3×3×4 =36
    1 0 2 4×3×3 =36
    1 1 1 4×4×4 =64
    1 2 0 4×3×3 =36
    2 0 1 3×3×4 =36
    2 1 0 3×4×3 =36
    2 2 2 3×3×3 =27

    which means the total is 2(27)+64+6(36)=54+64+216=334

    Answer: 334

  23. We begin with a diagram. Suppose C has coordinates (a,b) and is the vertex of the rectangle to the right of B and on the line x+2y18=0. We let E be the point with coordinates (a,2) and F be the point with x-coordinate a and lying on the line through B and D.

    The area of rectangle ABCD will be twice that of BCD, and we can compute the area of BCD by subtracting the areas of BEC and CFD from the area of BEF.
    To compute these areas, we need to find the values of a, b, and c.
    First, we find the equation of the line through B and D. Since this line passes through (4,2) and (12,8), it has slope 82124=68=34. Thus, its equation is of the form y=34x+k for some k. Substituting (x,y)=(4,2), we have 2=34(4)+k so k=1, which means the equation of the line is y=34x1.
    To find the coordinates of C, we note that the point of intersection of the diagonals of a rectangle is equidistant from all four of its vertices.
    The intersection of the diagonals will be the intersection of the lines y=34x1 and x+2y18=0. Substituting, we have x+2(34x1)=18 or x+32x2=18. Multiplying through by 2, we have 2x+3x=40, so x=8 from which we get y=34(8)1=5. The coordinates of the centre of the rectangle are (8,5). The distance from this point to point B is (84)2+(52)2=16+9=25=5. Therefore, C is the point on the line x+2y=18 that is 5 units away from (8,5). The condition on the distance gives us (a8)2+(b5)2=5 or (a8)2+(b5)2=25. Since (a,b) is on x+2y=18, we also have a+2b=18 which rearranges to a8=102b=2(5b). Substituting leads to [2(5b)]2+(b5)2=25 which can be rearranged to get 5(b5)2=25. Thus, (b5)2=5, so b=5±5. The point C is to the right of the centre of the rectangle, and since the line connecting A and C has negative slope, this means b should be less than 5. Therefore, b=55. Substituting into a+2b=18, we have a=182(55)=8+25.
    We now know the coordinates of C, and therefore, the coordinates of E are (8+25,2). To find the y-coordinate of F, we need only plug the known value of a in for x in the equation y=34x1 to get c=34(8+25)1=5+325.
    The area of BEF is 12BEEF=12(a4)(c2)=12(8+254)(5+3252)=(2+5)(3+325)=6+35+35+152=272+65

    The area of BEC is 12BEEC=12(a4)(b2)=12(8+254)(552)=(2+5)(35)=625+355=1+5

    Finally, since the height of CFD is the horizontal distance from D to FC which is a12, we have that the area of CFD is 12FC(a12)=12(cb)(a12)=12(5+325(55))(8+2512)=12(525)(4+25)=55+252

    Using these three ares, we can compute the area of BCD as (272+65)(1+5)(55+252)=105 Thus, the area of rectangle ABCD is 2(105)=205.

    Answer: 205

  24. By clearing denominators, the equation becomes 2x(x2+x+3)+3x(x2+5x+3)=(x2+5x+3)(x2+x+3) which can be expanded to get 2x3+2x2+6x+3x3+15x2+9x=x4+6x3+11x2+18x+9 and then rearranged to x4+x36x2+3x+9=0 Recognizing that x=3 is a solution to the equation above, x+3 must be a factor of the polynomial on the left, so we can factor to get (x+3)(x32x2+3)=0 Next, x=1 is a root of x32x2+3, so we can further factor x+1 from the cubic on the left side of the above equation to get (x+3)(x+1)(x23x+3)=0 Using the quadratic formula, the roots of the polynomial x23x+3 are x=3+(3)24(3)2=3+32and332 neither of which are real. Therefore, the only real values of x that can possibly satisfy the equation are 3 and 1. It is easily check that both x=1 and x=3 satisfy the original equation, so the answer is (1)+(3)=4.

    Answer: 4

  25. We will suppose O is at the origin, which gives rise to coordinates A(30,30), B(30,30), C(30,30), and D(30,30). Additionally, we will have E(30,0).

    The line passing through A and C also passes through the origin and has slope m=303030(30)=6060=1, and hence has equation y=x. The line through B and E has slope m=30030(30)=3060=12 and passes through (30,30), so if its equation is y=12x+b for some real number b. Substituting (x,y)=(30,30), we have 30=12(30)+b or b=15. Thus, the line passing through both B and E has equation y=12x+15.

    We can now compute the coordinates of points G and H. Since H is the intersection of line segments AC and BE, the x-coordinate of H satisfies x=12x+15 or 15=32x, which can be solved to get x=10. Since H lies on the line y=x, this means H has coordinates (10,10).

    The circle has diameter equal to the side length of the square, which is given to be 60. Therefore, its radius is 602=30, which means the equation of the circle is x2+y2=302. Points G and E are the points of intersection with the circle and the line y=12x+15, which means their x-coordinates can be found by substituting 12x+15 in the equation of the circle for y and solving for x. That is, the x-coordinates of E and G are the solutions to the equation x2+(12x+15)2=900 which can be expanded to get x2+14x2+15x+225=900. Multiplying through by 4 and rearranging, we get 5x2+60x2700=0 and after dividing through by 5 we have x2+12x540=0. We know that the x-coordinate of E is a root, which means x+30 must be a factor of the polynomial on the right, which leads to (x+30)(x18)=0. Thus, the other solution is x=18, so the x-coordinate of G must be 18. Substituting into y=12x+15, we get that y=182+15=24, so G has coordinates (18,24).

    We next observe that the shaded region bund by the circle and the line segments AF and AB is congruent to the region bound by the circle, AF, and AE. Thus, the shaded region is equal to the sum of the areas of HGO and AHE.

    If we take AE as the base of AHE, then its height will be the horizontal distance from H to line segment AE. This is equal to the difference between the x coordinates of A and H, or 10(30)=20. Thus, the area of AHE is 12(AE)(20)=10AE=10(30)=300.

    To compute the area of HGO, we will subtract the area of EHO from the area of EGO. Taking EO as a common base, the heights of EHO and EGO are the y-coordinates of H and G, respectively. These values are 10 and 24, so the area of HGO is

    12(EO)(24)12(EO)(10)=12(EO)(2410)=12(30)(14)=210

    Therefore, the total area of the shaded regions is 300+210=510.

    Answer: 510

Relay Problems

(Note: Where possible, the solutions to parts (b) and (c) of each Relay are written as if the value of t is not initially known, and then t is substituted at the end.)

    1. Evaluating, 2+5×53=2+253=273=9.

    2. The area of a triangle with base 2t and height 2t6 is 12(2t)(2t6) or t(2t6).
      The answer to (a) is 9, so t=9 which means t(2t6)=9(12)=108.

    3. Since ABC is isosceles with AB=BC, it is also true that BCA=BAC.
      The angles in a triangle sum to 180, so 180=ABC+BAC+BCA=ABC+2BAC=t+2BAC Therefore, BAC=12(180t)=12(180108)=12(72)=36.

    Answer: 9,108,36

    1. In an equilateral triangle, all sides of are of equal length. This means x+5=14 and y+11=14. Solving these equations for x and y, respectively, we get x=145=9 and y=1411=3. Therefore, x+y=9+3=12.

    2. Let k be the number of $1 coins that Gray has. It is given that k is also the number of $2 coins, which means the total amount of money that Gray has is k($1)+k($2)=k($1+$2)=k($3) Therefore, $3k=$t so k=t3=123=4.

    3. In the beginning, Elsie has tx applies. After giving away 10% of her apples, she still has 90% of the original amount, or .9tx apples. She gives 6 of these apples to her sister and is left with 48 apples, which means .9tx6=48 or .9tx=54. Multiplying both sides by 109 gives 109910tx=109(54)=60 so tx=60 which means x=60t. With t=4, we have x=604=15.

    Answer: 12,4,15

    1. There are four numbers in total, so their average is (x+5)+14+x+54=2x+244=x+122 The average is 9, so x+122=9 or x+12=18 which means x=6.

    2. Adding the equations x+ty+8=0 and 5xty+4=0, we get x+ty+8+5xty+4=0+0 or 6x+12=0. Solving for x gives x=2. Substituting into the first equation gives 2+ty+8=0 or ty=6 which means y=6t. Substituting the values of x and y into the third equation and rearranging, we have 3(2)k(6t)+1=06+6kt=16kt=5k=5t6 Using t=6, we get that k=5×66=5.

    3. Using the information given, the quadrilateral looks like

      The sides AB and CD are parallel, so the quadrilateral is a trapezoid with parallel sides CD=10, AB=k3, and height h=t. We are given that k>3, so k3 is positive and makes sense as a length.

      The area of the trapezoid is h2(AB+CD)=t2(k3+10)=t2(k+7) Since the area is 50, this means 50=t2(k+7) or k+7=100t so k=100t7. Using that t=5, we have k=10057=207=13.

    Answer: 6,5,13

    1. Since 2020=5×404, the multiples of 5 less than 2020 are 5×1,5×2,5×3,,5×403,5×404 so there are 404 multiples of 5 less than or equal to 2020. This means M=404. Similarly, the number of multiples of 2020 between 1 and 2020 is N=202020=101. Thus, 10M÷N=4040÷101=40.

    2. By alternating angles, BAC=t. This means BCA=1802tt=(1803t). Again by alternating angles, DCE=BCA, so x=180CDEDCE=18090(1803t)=3t90 which means x=3t90=3×4090=30.

    3. Let A be the number of adults and C be the number of children. The information given translates into the two equations: A+C=t and 9A+5C=190. Rearranging the first equation, we have A=tC and substituting this into the second equation gives 9(tC)+5C=190 or 4C=9t190 which means C=9t1904. We have that t=30, so C=9(30)1904=804=20.

    Answer: 40,30,20