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2018 Fermat Contest
Solutions
(Grade 11)

Tuesday, February 27, 2018
(in North America and South America)

Wednesday, February 28, 2018
(outside of North American and South America)

©2017 University of Waterloo


  1. Evaluating, 20162017+20182019+2020=2016+(20182017)+(20202019)=2016+1+1=2018

    Answer: (D)

  2. Since the maximum temperature was 14C and the minimum temperature was 11C, then the range of temperatures was 14C(11C)=25C.

    Answer: (B)

  3. The expression (3x+2y)(3x2y) is equal to 3x+2y3x+2y which equals 4y.
    When x=2 and y=1, this equals 4(1) or 4.

    Answer: (A)

  4. The fraction 57 is between 0 and 1.
    The fraction 283 is equivalent to 913 and so is between 9 and 10.
    Therefore, the integers between these two fractions are 1,2,3,4,5,6,7,8,9, of which there are 9.

    Answer: (B)

  5. If =1, then =××=1×1×1=1, which is not possible since and must be different positive integers.
    If =2, then =××=2×2×2=8, which is possible.
    If =3, then =××=3×3×3=27, which is not possible since is less than 20.
    If is greater than 3, then will be greater than 27 and so cannot be greater than 3.
    Thus, =2 and so =8.
    This means that ×=8×8=64.

    Answer: (D)

  6. Since QRT=158, then QRP=180QRT=180158=22.
    Since PRS=QRS and QRP=PRS+QRS, then QRS=12QRP=12(22)=11.
    Since QSR is right-angled at Q, then QSR=18090QRS=9011=79.

    Answer:(E)

  7. Since Bev has driven 312 km and still has 858 km left to drive, the distance from Waterloo to Marathon is 312 km+858 km=1170 km.
    The halfway point of the drive is 12(1170 km)=585 km from Waterloo.
    To reach this point, she still needs to drive 585 km312 km=273 km.

    Answer: (B)

  8. A line segment joining two points is parallel to the x-axis exactly when the y-coordinates of the two points are equal.
    Here, this means that 2k+1=4k5 and so 6=2k or k=3.
    (We can check that when k=3, the coordinates of the points are (3,7) and (8,7).)

    Answer: (A)

  9. Since the area of rectangle PQRS is 180 and SR=15, then PS=18015=12.
    Since PS=12 and US=4, then PU=PSUS=124=8.
    Since PUT is right-angled at U, then by the Pythagorean Theorem, TU=PT2PU2=10282=36=6 since TU>0.
    In PTS, we can consider base PS and height TU.
    Therefore, its area is 12(PS)(TU)=12(12)(6)=36.

    Answer: (B)

  10. For any real number x not equal to 0, x2>0.
    Since 1<x<0, then x2<(1)2=1, and so 0<x2<1.
    Of the given points, only C is between 0 and 1.

    Answer: (C)

  11. Since 56 of the balls are white and the remainder of the balls are red, then 16 of the balls are red.
    Since the 8 red balls represent 16 of the total number of balls and 56=516, then the number of white balls is 58=40.

    Answer: (C)

  12. There is 1 square that is 1×1 that contains the shaded square (namely, the square itself).
    There are 4 squares of each of the sizes 2×2, 3×3 and 4×4 that contain the shaded square.

    Four squares of size 2x2, 3x3 and 4x4 are nested within each other, such that two sides of each square meet, starting at the top left square. The nested squares have shifted 1 block to the right in the larger square. The nested squares have shifted 1 block down in the larger square from the last image. The nested squares have shifted 1 block to the right in the larger square from the last image.

    Finally, there is 1 square that is 5×5 that contains the shaded square (namely, the 5×5 grid itself).
    In total, there are thus 1+4+4+4+1=14 squares that contain the shaded 1×1 square.

    Answer:(E)

  13. We would like to find the first time after 4:56 where the digits are consecutive digits in increasing order.
    It would make sense to try 5:67, but this is not a valid time.
    Similarly, the time cannot start with 6, 7, 8 or 9.
    No time starting with 10 or 11 starts with consecutive increasing digits.
    Starting with 12, we obtain the time 12:34. This is the first such time.
    We need to determine the length of time between 4:56 and 12:34.
    From 4:56 to 11:56 is 7 hours, or 7×60=420 minutes.
    From 11:56 to 12:00 is 4 minutes.
    From 12:00 to 12:34 is 34 minutes.
    Therefore, from 4:56 to 12:34 is 420+4+34=458 minutes.

    Answer: (A)

  14. The line with equation y=x has slope 1 and passes through (0,0).
    When this line is translated, its slope does not change.
    When this line is translated 3 units to the right and 2 units down, every point on the line is translated 3 units to the right and 2 units down. Thus, the point (0,0) moves to (3,2).
    Therefore, the new line has slope 1 and passes through (3,2).
    Thus, its equation is y(2)=1(x3) or y+2=x3 or y=x5.
    The y-intercept of this line is 5.

    Answer: (C)

  15. Each entry in the grid must be a divisor of the product of the numbers in its row and the product of the numbers in its column.

    Only two of the products are multiples of 5, namely 160 and 135.
    This means that the 5 must go in the second row and third column.
    From this, we can see that the product of the other two numbers in the second row is 1355=27.
    Since all of the entries are between 1 and 9, then the remaining two numbers in this row must be 3 and 9.
    Since 9 is not a divisor of 21, then 9 must be in the middle column.
    This means that the product of the remaining numbers in the middle column is 1089=12.
    This means that the remaining digits in the middle column are 3 and 4, or 2 and 6. (These are the only factor pairs of 12 from the list of possible entries.)
    Since 3 already occurs in the second row, then the entries in the second column must be 2 and 6.
    Since 6 is not a divisor of 56, then 6 cannot go in the first row.
    This means that 6 goes in the third row and so N=6.
    We can complete the grid as follows:

    The first row has numebers 7, 4, 2, the second has 3, 9, 5 and the third has 1, 6, 8.

    Answer:(D)

  16. Solution 1
    If point R is placed so that PQ=QR=PR, then the resulting PQR is equilateral.
    Since points P and Q are fixed, then there are two possible equilateral triangles with PQ as a side – one on each side of PQ.

    R1 is situated above points P, Q, and R2 is situated below them.

    One way to see this is to recognize that there are two possible lines through P that make an angle of 60 with PQ.

    Solution 2
    Consider the line segment PQ. Draw a circle with centre P that passes through Q and a circle with centre Q that passes through P.

    The two circes intersect at R1 and R2.

    Suppose that the point R satisfies PQ=QR=PR.
    Since PQ=QR, then P and R are the same distance from Q, so R lies on the circle with centre Q that passes through P.
    Since PQ=PR, then R lies on the circle with centre P that passes through Q.
    In other words, point R is on both circles in the diagram.
    Since these two circles intersect in exactly two points, then there are two possible locations for R.

    Answer: (C)

  17. The side length of the square is 2 and M and N are midpoints of sides.
    Thus, SM=MR=QN=NR=1.
    Using the Pythagorean Theorem in PSM, we get PM=PS2+SM2=22+12=5 since PM>0.
    Similarly, PN=5.
    Using the Pythagorean Theorem in MNR, we get MN=MR2+NR2=12+12=2 since MN>0.
    Using the cosine law in PMN, we get MN2=PM2+PN22(PM)(PN)cos(MPN)2=5+52(5)(5)cos(MPN)2=1010cos(MPN)10cos(MPN)=8cos(MPN)=810=45

    Answer: (A)

  18. Suppose that 7+48=m+n.
    Squaring both sides, we obtain 7+48=(m+n)2.
    Since (m+n)2=m2+2mn+n, then 7+48=(m2+n)+2mn.
    Let’s make the assumption that m2+n=7 and 2mn=48.
    Squaring both sides of the second equation, we obtain 4m2n=48 or m2n=12.
    So we have m2+n=7 and m2n=12.
    By inspection, we might see that m=2 and n=3 is a solution.
    If we didn’t see this by inspection, we could note that n=7m2 and so m2(7m2)=12 or m47m2+12=0.
    Factoring, we get (m23)(m24)=0.
    Since m is an integer, then m23.
    Thus, m2=4 which gives m=±2. Since m is a positive integer, then m=2.
    When m=2, we get n=7m2=3.
    Therefore, m=2 and n=3, which gives m2+n2=13.
    We note that m+n=2+3 and that (2+3)2=4+43+3=7+43=7+48, as required. This means that, while the assumption we made at the beginning was not fully general, it did give us an answer to the problem.

    Answer:(E)

  19. Solution 1
    Over the first 3 minutes of the race, Peter ran 48 m farther than Radford. Here is why:

    We note that at a time of 0 minutes, Radford was at the 30 m mark.
    If Radford ran d m over these 3 minutes, then he will be at the (d+30) m mark after 3 minutes.
    Since Peter is 18 m ahead of Radford after 3 minutes, then Peter is at the (d+30+18) m mark.
    This means that, in 3 minutes, Peter ran (d+48) m which is 48 m farther than Radford’s d m.

    Since each runs at a constant speed, then Peter runs 48 m3 min=16 m/min faster than Radford.
    Since Peter finishes the race after 7 minutes, then Peter runs for another 4 minutes.
    Over these 4 minutes, he runs (4 min)(16 m/min)=64 m farther than Radford.
    After 3 minutes, Peter was 18 m ahead of Radford.
    Therefore, after 7 minutes, Peter is 18 m+64 m=82 m farther ahead than Radford, and so Radford is 82 m from the finish line.

    Solution 2
    As in Solution 1, suppose that Radford ran d m over the first 3 minutes and so Peter runs (d+48) m over these first 3 minutes.
    Since Peter’s speed is constant, he runs 43(d+48) m over the next 4 minutes.
    Since Radford’s speed is constant, he runs 43d over these next 4 minutes.
    This means that Peter runs a total of (d+48) m+43(d+48) m=73(d+48) m.
    Also, Radford is (30+d+43d) m from the start after 7 minutes, since he had a 30 m head start.
    Thus, Radford’s distance from the finish line, in metres, is 73(d+48)(30+d+43d)=73d+11230d43d=82

    Answer: (D)

  20. We count the positive integers x for which the product (x2)(x4)(x6)(x2016)(x2018)() equals 0 and is less than 0 separately.
    The product () equals 0 exactly when one of the factors equals 0.
    This occurs exactly when x equals one of 2,4,6,,2016,2018.
    These are the even integers from 2 to 2018, inclusive, and there are 20182=1009 such integers.
    The product () is less than 0 exactly when none of its factors is 0 and an odd number of its factors are negative.
    We note further that for every integer x we have x2>x4>x6>>x2016>x2018 When x=1, we have x2=1 and so all 1009 factors are negative, making () negative.
    When x=3, we have x2=1, x4=1 and all of the other factors are negative, giving 1008 negative factors and so a positive product.
    When x=5, we have x2=3, x4=1 and x6=1 and all of the other factors are negative, giving 1007 negative factors and so a negative product.
    This pattern continues giving a negative value for () for x=1,5,9,13,,2013,2017.
    There are 1+201714=505 such values (starting at 1, these occur every 4 integers).
    When x2019, each factor is positive and so () is positive.
    Therefore, there are 1009+505=1514 positive integers x for which the product () is less than or equal to 0.
    We should further justify the pattern that we found above.
    Suppose that x=4n+1 for n=0,1,2,,504. (These are the integers 1,5,9,13,,2017.)
    Then () becomes (4n1)(4n3)(4n5)(4n2015)(4n2017) The 2kth factor is (n(4k1)) and so when n=4k, this factor is positive and the next factor is negative.
    In other words, when n=2k, the first 2k of these factors are positive and the remaining factors are negative.
    In other words, when n=2k, there is an even number of positive factors.
    Since the total number of factors is 1009, which is odd, then the number of negative factors is odd and so the product is negative.
    In a similar way, we can show that if x=4n+3 for n=0,1,2,,503 (these are the integers 3,7,11,,2011,2015), then the product is positive.
    This confirms that this pattern continues.

    Answer: (C)

  21. Substituting n=1 into the equation an+1=an+an+21 gives a2=a1+a31.
    Since a1=x and a3=y, then a2=x+y1.
    Rearranging the given equation, we obtain an+2=an+1an+1 for each n1.
    Thus, a4=a3a2+1=y(x+y1)+1=2xa5=a4a3+1=(2x)y+1=3xya6=a5a4+1=(3xy)(2x)+1=2ya7=a6a5+1=(2y)(3xy)+1=xa8=a7a6+1=x(2y)+1=x+y1 Since a7=a1 and a8=a2 and each term in the sequence depends only on the previous two terms, then the sequence repeats each 6 terms.
    (For example, a9=a8a7+1=a2a1+1=a3 and so on.)
    Now a1+a2+a3+a4+a5+a6=x+(x+y1)+y+(2x)+(3xy)+(2y)=6 which means that the sum of each successive group of 6 terms is also equal to 6.
    We note that 2016=6336 and so the 2016th term is the end of a group of 6 terms, which means that the sum of the first 2016 terms in the sequence is 6336=2016.
    Finally, a2017=a1=x and a2018=a2=x+y1.
    Thus, the sum of the first 2018 terms is 2016+x+(x+y1)=2x+y+2015.

    Answer:(E)

  22. First, we find the coordinates of the points P and Q in terms of k by finding the points of intersection of the graphs with equations y=x2 and y=3kx+4k2.
    Equating values of y, we obtain x2=3kx+4k2 or x23kx4k2=0.
    We rewrite the left side as x24kx+kx+(4k)(k)=0 which allows us to factor and obtain (x4k)(x+k)=0 and so x=4k or x=k.
    Since k>0, P is in the second quadrant and Q is in the first quadrant, then P has x-coordinate k (which is negative).
    Since P lies on y=x2, then its y-coordinate is (k)2=k2 and so the coordinates of P are (k,k2).
    Since Q lies on y=x2 and has x-coordinate 4k, then its y-coordinate is (4k)2=16k2 and so the coordinates of Q are (4k,16k2).
    Our next step is to determine the area of OPQ in terms of k.
    Since the area of OPQ is numerically equal to 80, this will give us an equation for k which will allow us to find the slope of the line.
    To find the area of OPQ in terms of k, we drop perpendiculars from P and Q to S and T, respectively, on the x-axis.

    The area of OPQ is equal to the area of trapezoid PSTQ minus the areas of PSO and QTO.
    Trapezoid PSTQ has parallel bases SP and TQ and perpendicular height ST.
    Since the coordinates of P are (k,k2), then SP=k2.
    Since the coordinates of Q are (4k,16k2), then TQ=16k2.
    Also, ST=4k(k)=5k.
    Thus, the area of trapezoid PSTQ is 12(SP+TQ)(ST)=12(k2+16k2)(5k)=852k3.
    PSO is right-angled at S and so has area 12(SP)(SO)=12(k2)(0(k))=12k3.
    QTO is right-angled at T and so has area 12(TQ)(TO)=12(16k2)(4k0)=32k3.
    Combining these, the area of POQ equals 852k312k332k3=10k3.
    Since this area equals 80, then 10k3=80 or k3=8 and so k=2.
    This means that the slope of the line is 3k which equals 6.

    Answer: (D)

  23. We are told that (xa)(x6)+3=(x+b)(x+c) for all real numbers x.
    In particular, this equation holds when x=6.
    Substituting x=6 gives (6a)(66)+3=(6+b)(6+c) or 3=(6+b)(6+c).
    Since b and c are integers, then 6+b and 6+c are integers, which means that 6+b is a divisor of 3.
    Therefore, the possible values of 6+b are 3,1,1,3.
    These yield values for b of 3,5,7,9.
    We need to confirm that each of these values for b gives integer values for a and c.
    If b=3, then 6+b=3. The equation 3=(6+b)(6+c) tells us that 6+c=1 and so c=5.
    When b=3 and c=5, the original equation becomes (xa)(x6)+3=(x3)(x5).
    Expanding the right side gives (xa)(x6)+3=x28x+15 and so (xa)(x6)=x28x+12.
    The quadratic x28x+12 factors as (x2)(x6) and so a=2 and this equation is an identity that is true for all real numbers x.
    Similarly, if b=5, then c=3 and a=2. (This is because b and c are interchangeable in the original equation.)
    Also, if b=7, then c=9 and we can check that a=10.
    Similarly, if b=9, then c=7 and a=10.
    Therefore, the possible values of b are b=3,5,7,9.
    The sum of these values is (3)+(5)+(7)+(9)=24.

    Answer: (B)

  24. We use the notation “a/b/c” to mean a pucks in one bucket, b pucks in a second bucket, and c pucks in the third bucket, ignoring the order of the buckets.
    Yellow buckets
    1/0/0: With 1 puck to distribute, the distribution will always be 1/0/0.
    Blue buckets
    Since there are 2 pucks to distribute amongst the three buckets, then there is a total of 32=9 ways of doing this. (There are 3 possibilities for each of 2 pucks.)
    2/0/0: There are 3 ways in which the 2 pucks end up in the same bucket (1 way for each of the 3 buckets). The probability of this is 39.
    1/1/0: Thus, there are 93=6 ways in which the 2 pucks are distributed with 1 puck in each of two buckets and 0 pucks in the third bucket. The probability of this is 69.
    Red buckets
    With 3 pucks to distribute amongst 3 buckets, there is a total of 33=27 ways.
    3/0/0: There are 3 ways in which the 3 pucks end up in the same bucket (1 way for each of the 3 buckets). The probability of this is 327.
    1/1/1: There are 321=6 ways in which the 3 pucks end up with one in each bucket (3 choices of bucket for the first puck, 2 for the second, and 1 for the third). The probability of this is 627.
    2/1/0: Thus, there are 2736=18 ways in which the 2 pucks are distributed with 2 pucks in 1 bucket, 1 puck in 1 bucket, and 0 pucks in 1 bucket. The probability of this is 1827.
    Green buckets
    With 4 pucks to distribute amongst 3 buckets, there is a total of 34=81 ways.
    4/0/0: There are 3 ways in which the 4 pucks end up in the same bucket (1 way for each of the 3 buckets). The probability of this is 381.
    3/1/0: There are 4×3×2=24 ways in which the pucks end up with 3 in one bucket and 1 in another (4 ways to choose a puck to be on its own, 3 ways to choose the bucket for this puck, and 2 ways to choose the bucket for the 3 pucks). The probability of this is 2481.
    2/1/1: There are 6 ways of choosing two of the four pucks. (If they are labelled W, X, Y, Z, then we can choose WX, WY, WZ, XY, XZ, or YZ.) There are 6×3×2=36 ways in which the pucks can be distributed with 2 pucks in one bucket and 1 puck in each of the remaining buckets (6 ways to choose the 2 pucks that go together, 3 ways to choose the bucket, and 2 ways in which the remaining 2 pucks can be assigned to the remaining 2 buckets). The probability of this is 3681.
    2/2/0: Thus, there are 8132436=18 ways in which the 4 pucks are distributed with 2 pucks in each of 2 buckets. The probability of this is 1881.
    For a green bucket to contain more pucks than each of the other 11 buckets, the following possible distributions exist with probabilities as shown:

    Green Red Blue Yellow Probability
    4/0/0 (p=381) Any (p=1) Any (p=1) Any (p=1) 381
    3/1/0 (p=2481) Any but 3/0/0 (p=1327) Any (p=1) Any (p=1) 24812427
    2/1/1 (p=3681) 1/1/1 (p=627) 1/1/0 (p=69) Any (p=1) 368162769

    A 2/2/0 distribution of pucks among green buckets cannot satisfy the desired conditions because there would be not be a single green bucket with more pucks in it than any other bucket, as there would be two green buckets containing the same number of pucks.
    Therefore, the overall probability is 381+24812427+368162769=127+82789+492923=9243+64243+16243=89243.

    Answer: (B)

  25. Suppose that D is a digit and k is a positive integer. Then D(k)=DDDDk times=D1111k times=D199999k times=D19(10000k times1)=D19(10k1) Therefore, the following equations are equivalent: P(2k)Q(k)=(R(k))2P19(102k1)Q19(10k1)=(R19(10k1))2P19(102k1)Q19(10k1)=R2181(10k1)29P(102k1)9Q(10k1)=R2(10k1)29P(10k1)(10k+1)9Q(10k1)=R2(10k1)29P(10k+1)9Q=R2(10k1)(since 10k10)9P10k+9K9Q=R210kR29P9Q+R2=10k(R29P) We consider three cases: 3k2018, k=1, and k=2.
    Case 1: 3k2018
    Suppose that R29P0.
    Since k3, then 10k(R29P)>1000 if R29P>0 and 10k(R29P)<1000 if R29P<0.
    Since P,Q,R are digits, then 9P9Q+R2 is at most 9(9)9(0)+92=162 and 9P9Q+R2 is at least 9(0)9(9)+02=81.
    This means that if R29P0, we cannot have 9P9Q+R2=10k(R29P) since the possible values do not overlap.
    So if 3k2018, we must have R29P=0 and so 9P9Q+R2=0.
    If R2=9P, then R2 is a multiple of 3 and so R is a multiple of 3.
    Since R is a positive digit, then R=3 or R=6 or R=9.
    If R=3, then 9P=R2=9 and so P=1.
    Since 9P9Q+R2=0, then 9Q=9(1)+9=18 and so Q=2.
    If R=6, then 9P=R2=36 and so P=4.
    Since 9P9Q+R2=0, then 9Q=9(4)+36=72 and so Q=8.
    If R=9, then 9P=R2=81 and so P=9.
    Since 9P9Q+R2=0, then 9Q=9(9)+81=162 and so Q cannot be a digit.
    Therefore, in the case where 3k2018, we obtain the quadruples (P,Q,R,k)=(1,2,3,k) and (P,Q,R,k)=(4,8,9,k).
    Since there are 20183+1=2016 possible values of k, then we have 22016=4032 quadruples so far.
    Case 2: k=1
    Here, the equation 9P9Q+R2=10k(R29P) becomes 9P9Q+R2=10R290P or 99P=9R2+9Q or 11P=R2+Q.
    For each possible value of P from 1 to 9, we determine the possible values of Q and R by looking for perfect squares that are at most 9 less than 11P.
    P=1: Here, 11P=11 which is close to squares 4 and 9. We obtain (R,Q)=(2,7),(3,2).
    P=2: Here, 11P=22 which is close to the square 16. We obtain (R,Q)=(4,6).
    P=3: Here, 11P=33 which is close to the square 25. We obtain (R,Q)=(5,8).
    P=4: Here, 11P=44 which is close to the square 36. We obtain (R,Q)=(6,8).
    P=5: Here, 11P=55 which is close to the square 49. We obtain (R,Q)=(7,6).
    P=6: Here, 11P=66 which is close to the square 64. We obtain (R,Q)=(8,2).
    P=7: There are no perfect squares between 68 and 76, inclusive.
    P=8: Here, 11P=88 which is close to the square 81. We obtain (R,Q)=(9,7).
    P=9: There are no perfect squares between 90 and 98, inclusive.
    Since k=1 in each of these cases, we obtain an additional 8 quadruples.
    Case 3: k=2
    Here, the equation 9P9Q+R2=10k(R29P) becomes 9P9Q+R2=100R2900P or 909P=99R2+9Q or 101P=11R2+Q.
    As P ranges from 1 to 9, the possible values of 101P are 101,202,303,404,505,606,707,808,909.
    As R ranges from 1 to 9, the possible values of 11R2 are 11,44,99,176,275,396,539,704,891.
    The pairs of integers in the first and second lists that differ by at most 9 are

    Since k=2 in each of these cases, we obtain an additional 3 quadruples.
    In total, there are thus N=4032+8+3=4043 quadruples.
    The sum of the digits of N is 4+0+4+3=11.

    Answer: (C)