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2022 Fermat Contest
Solutions
(Grade 11)

Wednesday, February 23, 2022
(in North America and South America)

Thursday, February 24, 2022
(outside of North American and South America)

©2021 University of Waterloo


  1. Evaluating, 6+(3×6)12=6+1812=12.

    Answer: (C)

  2. Solution 1

    Since the average of two numbers is 7, their sum is 2×7=14.
    Since one of the numbers is 5, the other is 145=9.

    Solution 2

    The average of two numbers is 7 and one of the numbers is 5.
    Since 5 is 2 less than 7, the other number must be 2 more than 7, and so is 9.

    Answer: (E)

  3. From the day on which she walks 500 m to the day on which she walks 4500 m, Gauravi increases her distance by (4500 m)(500 m)=4000 m.
    Since Gauravi increases her distance by 500 m each day, then it takes 4000 m500 m=8 days to increase from 500 m to 4500 m.
    Starting from Monday and counting forward by 8 days (which is 1 week and 1 day) gets to Tuesday, and so Gauravi walks exactly 4500 m on a Tuesday.

    Answer: (C)

  4. By arranging 4 rows of 4 squares of side length 2, a square of side length 8 can be formed.

    Thus, 44=16 squares can be arranged in this way. Since these smaller squares completely cover the larger square, it is impossible to use more 2×2 squares, so 16 is the largest possible number.

    Answer: (C)

  5. Since the list includes 15 integers, then an integer has a probability of 13 of being selected if it occurs 1315=5 times in the list.
    The integer 5 occurs 5 times in the list and no other integer occurs 5 times, so n=5.

    Answer: (E)

  6. The given triangle can be considered to have base PQ (which is vertical along the line x=2) and perpendicular height which runs from R horizontally to PQ.
    The line segment joining P(2,6) to Q(2,2) has length 4.
    Point R(8,5) is 6 units to the right of the line with equation x=2.
    Thus, PQR has area 1246=12.

    Answer: (D)

  7. Evaluating, (1+2+3)(1+12+13)=6(1+12+13)=6+62+63=6+3+2=11.

    Answer: (B)

  8. Since 10x+y=75 and 10y+x=57, then (10x+y)+(10y+x)=75+57 and so 11x+11y=132 Dividing by 11, we get x+y=12.
    (We could have noticed initially that (x,y)=(7,5) is a pair that satisfies the two equations, thence concluding that x+y=12.)

    Answer: (A)

  9. Since Pearl digs 4 holes in 7 days and 217=3, then in 21 days, Pearl digs 34=12 holes.

    Since Miguel digs 2 holes in 3 days and 213=7, then in 21 days, Miguel digs 72=14 holes.

    In total, they dig 12+14=26 holes in 21 days.

    Answer: (D)

  10. Manipulating the left side, 211×65=211×(2×3)5=211×25×35=216×35.
    Since 4x×3y=216×35 and x and y are positive integers, then y=5 (because 4x has no factors of 3).
    This also means that 4x=216.
    Since 4x=(22)x=22x, then 4x=216 gives 22x=216 and so 2x=16 or x=8.
    Therefore, x+y=8+5=13.

    Answer: (D)

  11. We use A, B, C, D, and E to represent Andy, Bev, Cao, Dhruv, and Elcim, respectively.
    We use the notation D>B to represent the fact “Dhruv is older than Bev”.
    The five sentences give D>B and B>E and A>E and B>A and C>B. These show us that Dhruv and Cao are older than Bev, and Elcim and Andy are younger than Bev.
    This means that two people are older than Bev and two people are younger than Bev, which means that Bev must be the third oldest.

    Answer: (B)

  12. Since d is an odd integer, then d+d is even and d×d is odd.
    Since e is an even integer, then e+e is even, which means that (e+e)×d is even.
    Also, e+d is odd, which means that d×(e+d) is odd.
    Thus, 2 of the 4 expressions are equal to an odd integer.

    Answer: (C)

  13. Suppose that each of the small rectangles has shorter sides of length x.
    Then the height of the rectangle in Figure A is 2x, which means that the longer side of each small rectangle has length 2x.
    Therefore, the rectangle in Figure A has height 2x and width 2x+x=3x, which means that its perimeter is 2(3x+2x)=10x. Also, the rectangle in Figure B has height 2x and width x+2x+x=4x, so its perimeter is 2(4x+2x)=12x.

    Therefore, the ratio of the perimeter of Figure A to the perimeter of Figure B is 10x:12x which is equal to 5:6, since x0.

    Answer: (E)

  14. Zebadiah must remove at least 3 shirts.
    If he removes 3 shirts, he might remove 2 red shirts and 1 blue shirt.
    If he removes 4 shirts, he might remove 2 red shirts and 2 blue shirts.
    Therefore, if he removes fewer than 5 shirts, it is not guaranteed that he removes either 3 of the same colour or 3 of different colours.
    Suppose that he removes 5 shirts. If 3 are of the same colour, the requirements are satisfied.
    If no 3 of the 5 shirts are of the same colour, then at most 2 are of each colour (for example, 2 red, 2 blue and 1 green). This means that he must remove shirts of 3 colours, since if he only removed shirts of 2 colours, he would remove at most 2+2=4 shirts.
    In other words, if he removes 5 shirts, it is guaranteed that there are either 3 of the same colour or shirts of all 3 colours.
    Thus, the minimum number is 5.

    Answer: (D)

  15. If a is odd, the output is a+3, which is even because it is the sum of two odd integers.
    If a is even, the output is a+5, which is odd, because it is the sum of an even integer and an odd integer.
    Starting with a=15 and using the machine 2 times, we obtain 1515+3=1818+5=23.
    Starting with 23 and using the machine 2 times, we obtain 2323+3=2626+5=31.
    Starting with an odd integer and using the machine 2 times, the net result is adding 8 to the input, because the odd input generates a first output that is 3 larger (and so even) and a second output that is 5 larger than the first output.
    This generates a net result that is 3+5 larger than the input.
    Therefore, using the machine 46 more times (that is, repeating the 2 steps a total of 23 more times), we add 8 a total of 23 more times to obtain the output 31+238=215.
    To this point, the machine has been used 50 times.
    Using the machine for the 51st time, 215215+3=218 and so the final output is 218.

    Answer: (B)

  16. Since the remainder when 111 is divided by n is 6, then 1116=105 is a multiple of n and n>6 (since, by definition, the remainder must be less than the divisor).
    Since 105=357, the positive divisors of 105 are 1,3,5,7,15,21,35,105.
    Therefore, the possible values of n are 7, 15, 21, 35, 105, of which there are 5.

    Answer: (A)

  17. Suppose that the original can has radius r cm and height h cm.
    Since the surface area of the original can is 300 cm2, then 2πr2+2πrh=300.
    When the radius of the original can is doubled, its new radius is 2r cm, and so an expression for its surface area, in cm2, is 2π(2r)2+2π(2r)h which equals 8πr2+4πrh, and so 8πr2+4πrh=900.
    When the height of the original can is doubled, its new height is 2h cm, and so an expression for its surface area, in cm2, is 2πr2+2πr(2h) which equals 2πr2+4πrh.
    Multiplying 2πr2+2πrh=300 by 3, we obtain 6πr2+6πrh=900.
    Since 8πr2+4πrh=900, we obtain 6πr2+6πrh=8πr2+4πrh2πrh=2πr2πrh=πr2 Since 2πr2+2πrh=300 and πrh=πr2, then 2πr2+2πr2=300 and so 4πr2=300 or πr2=75.
    Since πrh=πr2=75, then 2πr2+4πrh=675=450, and so the surface area of the cylinder with its height doubled is 450 cm2.

    Answer: (A)

  18. Let A be Aria’s starting point, B be Bianca’s starting point, and M be their meeting point.
    It takes Aria 42 minutes to walk from A to M and 28 minutes from M to B.
    (Note that 9:00 a.m. is 18 minutes after 8:42 a.m., and 9:10 a.m. is 10 minutes after 9:00 a.m..)
    Since Aria walks at a constant speed, then the ratio of the distance AM to the distance MB is equal to the ratio of times, or 42:28, which is equivalent to 3:2.
    Since it takes 42 minutes for Bianca to walk from B to M, the ratio of distances AM to MB is 3:2, and Bianca walks at a constant speed, then it takes Bianca 32×42=63 minutes to walk from M to A.
    Therefore, Bianca arrives at Aria’s starting point at 9:45 a.m.
    (Note that 9:00 a.m. is 18 minutes after 8:42 a.m., and 9:45 a.m. is 45 minutes after 9:00 a.m..)

    Answer: (D)

  19. Since PQR is right-angled at R, then by the Pythagorean Theorem, PQ2=PR2+QR2=122+162=144+256=400 Since PQ>0, then PQ=20.
    Since M is the midpoint of PQ, then MQ=12PQ=10.
    Now NMQ is similar to PRQ, since each is right-angled and they share a common angle at Q.
    Therefore, NQPQ=MQRQ and so NQ20=1016 which gives NQ=201016=252.
    Thus, RN=RQNQ=16252=322252=72.
    Since PNR is right-angled at R, its area equals 12PRRN=121272=21.

    Answer: (A)

  20. We note that t1=1113=230.67t1+t2=(1113)+(1214)=23+14=11120.92t1+t2+t3=(1113)+(1214)+(1315)=11+12+13131415=11+121415=1.05t1+t2+t3+t4=(1113)+(1214)+(1315)+(1416)=11+12+13+1413141516=11+1215161.13 This means that the sum of the first k terms is less than 1.499 for k=1,2,3,4.
    When k>4, we can extend the pattern that we saw for k=3 and k=4 to note that t1+t2+t3++tk1+tk=(1113)+(1214)+(1315)++(1k11k+1)+(1k1k+2)=11+12+13++1k1+1k1314151k+11k+2=11+12+1313++1k11k1+1k1k1k+11k+2=11+121k+11k+2=1.5001k+11k+2 This means that the sum of the first k terms is less than 1.499 exactly when 1k+1+1k+2 is greater than 0.001.
    As k increases from 4, each of 1k+1 and 1k+2 decreases, which means that their sum decreases as well.
    When k=1998, 1k+1+1k+2=11999+12000>12000+12000=11000=0.001.
    When k=1999, 1k+1+1k+2=12000+12001<12000+12000=11000=0.001.
    This means that 1k+1+1k+2 is greater than 0.001 exactly when k1998 and is less than 0.001 when k1999.
    In other words, the sum of the first k terms is less than 1.499 for k=1,2,3,4 as well as for 5k1998, which is the same as saying that this is true for 1k1998.
    Therefore, k=1998 is the largest positive integer for which the sum of the first k terms is less than 1.499.

    Answer: (E)

  21. The total mass of the six steel bars in the bags is at least 1+2+3+4+5+6=21 kg and at most 10+11+12+13+14+15=75 kg. This is because the masses of the 15 given bars are 1 kg, 2 kg, 3 kg, , 14 kg, and 15 kg.
    Since the six bars are divided between three bags with the same total mass in each bag, then the total mass in each bag is at least 21÷3=7 kg and at most 75÷3=25 kg.
    There are 257+1=19 masses that are an integer number of kilograms in this range (7 kg, 8 kg, 9 kg, , 23 kg, 24 kg, 25 kg).
    Each of 19 these masses is indeed possible. To see this, we note that

    which shows that 7, 8 and 9 are possible values of M.
    Continuing to increase the larger values to 15, 14, 13, we eventually obtain 1+15=2+14=3+13=16 and also that each value of M between 10 and 16, inclusive, will be a possible value of M.
    Now, we increase the smaller values, starting from the last three pairs:

    which shows that 17, 18, 19, 20, 21, 22, 23, 24, and 25 are also possible values of M.
    This shows that every integer value of M with 7M25 is possible.
    In summary, there are 19 possible values of M.

    Answer: 19

  22. We enclose the given rectangle in a larger rectangle with horizontal and vertical sides so that the vertices of the smaller rectangle lie on the sides of the larger rectangle.
    We will also remove the units from the problem and deal with dimensionless quantities.

    The larger rectangle has vertices A, B, C, and D, starting with A in the top right corner and moving clockwise around the perimeter. The enclosed rectangle has vertices V, W, Y, and Z with V lying on vertical right side AB, W on bottom horizontal side BC, Y on left vertical side CD, and Z on top horizontal side DA

    Since VWYZ is a rectangle, then YW=ZV=100 and ZY=VW=150.
    Since YCW is right-angled at C, by the Pythagorean Theorem, CW2=YW2YC2=1002202=10000400=9600 Since CW>0, then CW=9600=16006=16006=406.
    The height of Z above the horizontal line is equal to the length of DC, which equals DY+YC which equals DY+20.
    Now ZDY is right-angled at D and YCW is right-angled at C.
    Also, DYZ+ZYW+WYC=180, which means that DYZ+WYC=90, since ZYW=90.
    Since CWY+WYC=90 as well (using the sum of the angles in YCW), we obtain DYZ=CWY, which tells us that ZDY is similar to YCW.

    Therefore, DYZY=CWYW and so DY=ZYCWYW=150406100=606.

    Finally, DC=DY+20=606+20166.97. Rounded to the nearest integer, DC is 167.
    Since the length of DC to the nearest integer is 100+x and this must equal 167, then x=67.

    Answer: 67

  23. Suppose that k is a fixed, but unknown, positive integer.
    Suppose also that the lines with equations 9x+4y=600 and kx4y=24 intersect at the point with positive integer coordinates (x,y).
    Since 9x+4y=600 and kx4y=24, adding these equations, we get 9x+kx=624 and so (9+k)x=624.
    Since x and y are to be positive integers and k>0, then 9+k and x are a positive divisor pair of 624 with 9+k>9.
    Now 624=6104=6813=2431131, and so the positive divisors of 624 are 1,2,3,4,6,8,12,13,16,24,26,39,48,52,78,104,156,208,312,624 We also want the value of y to be a positive integer.
    Since the point (x,y) lies on the line with equation 9x+4y=600, then 4y=6009x which gives y=15094x, which is an integer exactly when x is a multiple of 4.
    Therefore, we want x to be a positive divisor of 624 which is a multiple of 4.
    Thus, the possible values of x are 4,8,12,16,24,48,52,104,156,208,312,624.
    The corresponding values of 9+k are 156,78,52,39,26,13,12,6,4,3,2,1.
    Since 9+k>9, we eliminate 6,4,3,2,1 from this list.
    Thus, the possible values of 9+k are 156,78,52,39,26,13,12.
    The corresponding values of k are 147,69,43,30,17,4,3.
    These correspond to the following values of x: 4,8,12,16,24,48,52.
    Using y=15094x, these give the following values of y: 141,132,123,114,96,42,33. These are indeed all positive.
    This means that there are 7 values of k with the required properties.

    Answer: 07

  24. Since f(p)=17, then ap2+bp+c=17.
    Since f(q)=17, then aq2+bq+c=17.
    Subtracting these two equations, we obtain a(p2q2)+b(pq)=0.
    Since p2q2=(pq)(p+q), this becomes a(pq)(p+q)+b(pq)=0.
    Since p<q, then pq0, so we divide by pq to get a(p+q)+b=0.
    Since f(p+q)=47, then a(p+q)2+b(p+q)+c=47 and so (p+q)(a(p+q)+b)+c=47.
    Since a(p+q)+b=0, then (p+q)(0)+c=47 which tells us that c=47.
    Since ap2+bp+c=17, then ap2+bp=30 and so p(ap+b)=30.
    Similarly, q(aq+b)=30.
    Since p and q are prime numbers and a and b are integers, then p and q must be prime divisors of 30. We note that 30=235 and also that p and q must be distinct.
    Since p<q, then p=2 and q=3, or p=2 and q=5, or p=3 and q=5.

    Alternatively, we could note that since f(p)=f(q)=17, then f(p)17=f(q)17=0.
    Therefore, f(x)17 is a quadratic polynomial with roots p and q, which means that we can write f(x)17=a(xp)(xq), since the quadratic polynomial has leading coefficient a.
    Since f(p+q)=47, then f(p+q)17=a(p+qp)(p+qq) which gives 4717=aqp or apq=30.
    As above, p=2 and q=3, or p=2 and q=5, or p=3 and q=5.

    If p=2 and q=3, the equations p(ap+b)=30 becomes 2(2a+b)=30 (or 2a+b=15) and the equation q(aq+b)=30 becomes 3(3a+b)=30 (or 3a+b=10).
    Subtracting 2a+b=15 from 3a+b=10, we obtain a=5 (note that a>0) which gives b=1525=25.
    Therefore, f(x)=5x225x+47.
    Since pq=6, then f(pq)=5(62)25(6)+47=77.

    If p=2 and q=5, we get 2a+b=15 and 5a+b=6.
    Subtracting the first of these from the second, we obtain 3a=9 which gives a=3 (note that a>0) and then b=1523=21.
    Therefore, f(x)=3x221x+47.
    Since pq=10, then f(pq)=3(102)21(10)+47=137.

    If p=3 and q=5, we get 3a+b=10 and 5a+b=6.
    Subtracting the first of these from the second, we obtain 2a=4 which gives a=2 (note that a>0) and then b=1032=16.
    Therefore, f(x)=2x216x+47.
    Since pq=15, then f(pq)=2(152)16(15)+47=257.

    The sum of these values of f(pq) is 77+137+257=471.
    The rightmost two digits of this integer are 71.

    Answer: 71

  25. Consider the grid as laid out in the problem: abcd5efgh We know that the sums of the integers along each row, along each column, and along the two main diagonals are all divisible by 5.
    We start by removing all but the integers 5, a, c, f, and h from the grid. ac5fh There are 9 choices for each of a and c.
    Since the sum of the entries on each diagonal is a multiple of 5, then a+5+h is a multiple of 5, which is equivalent to saying that a+h is a multiple of 5.
    Note that each of a and h is between 1 and 9.
    If a=1, then h=4 or h=9. If a=6, then h=4 or h=9.
    If a=2, then h=3 or h=8. If a=7, then h=3 or h=8.
    If a=3, then h=2 or h=7. If a=8, then h=2 or h=7.
    If a=4, then h=1 or h=6. If a=9, then h=1 or h=6.
    If a=5, then h=5.
    We write h=a to show that h depends on a. (Note that h does not depend on c.) We will remember later that there might be 1 or 2 possible values for h, depending on the value of a.

    Similarly, c+5+f is a multiple of 5, or equivalently that c+f is a multiple of 5 and this same combinations of possible values for c and f exist as for a and h.
    We write f=c. This gives us ac5ca Since a+b+c is a multiple of 5, then b+(a+c) is a multiple of 5. We write b=a+c, since b depends on a+c.
    This gives us aa+cc5ca Since a and c are each between 1 and 9, then a+c is between 2 and 18. Recall that b=a+c is also between 1 and 9.
    If a+c is one of 2, 7, 12, and 17, the possible values for b=a+c are 3 and 8.
    If a+c is one of 3, 8, 13, and 18, the possible values for b=a+c are 2 and 7.
    If a+c is one of 4, 9, and 14, the possible values for b=a+c are 1 and 6.
    If a+c is one of 5, 10, and 15, then b=a+c=5.
    If a+c is one of 6, 11, and 16, the possible values for b=a+c are 4 and 9.

    We can now start to consider a number of cases. Because we have seen above that the number of possibilities for some of the entries depend on whether or not a and c are 5, we look at (i) a=c=5, (ii) a=5 and c5, (iii) c=5 and a5, and (iv) a5 and c5.

    Case 1: a=c=5

    From above, there is only one choice for each of a and c: each must equal 5.
    Also, a+c=10 and so a+c must also equal 5, giving the grid: 555555 Similarly, each of the remaining cells can only be filled with 5, so there is only 1 way of completing the grid in this case.

    Case 2: a=5 and c5

    Since a=5, then a=5.
    Also, since a=5, then a+c=5+c which means that a+c is the same as c, giving the grid: 5cc5c5 Here, there are 8 choices for c (everything but 5) and 2 choices for each occurrence of c (since c is not 5).
    Furthermore, the possibilities for the 3 empty cells are determined by either the value of c or by the value of c, neither of which can be a multiple of 5.
    Thus, there are 2 possibilities for each of these 3 empty cells.
    Combining this information, in this case, there are thus 1282223=28 grids.

    Case 3: c=5 and a5

    If c=5 and a5, there are also 28 grids.

    Next, we consider the situation when a5 and c5.
    We know here that there are two possible values for each of a and c. However, the number of possible values for a+c depends on whether a+c is a multiple of 5. Additionally, the number of possibilities for the 3 unlabelled cells also depend on the values of combinations of a, c, a, and c. This leads to three more cases in which a5 and c5.

    Case 4: a5 and c5 and a+c is a multiple of 5

    There are 8 choices for a (everything but 5).
    There are then 2 choices for c (either of the choices that makes a+c a multiple of 5).
    There are 2 choices for each of a and c, since neither a nor c is 5.
    Also, a+c=5 since a+c is a multiple of 5.
    This gives the grid: a5c5ca The empty cell in the bottom row must be filled with a 5 to make the sum of the middle column a multiple of 5.
    We now examine the first and third columns and see that neither a+c nor c+a can be a multiple of 5.
    One way to justify this is to note that, since a+c is a multiple of 5, the remainders when a and c are divided by 5 must add to 5.
    This means that a and c have the same non-zero remainder when divided by 5, which in turn means that their sum is not divisible by 5.
    Therefore, the remaining 2 empty cells each have 2 possible entries to make their column sums multiples of 5.
    There are 8 choices for a, 2 choices for c, 2 cells which must be filled with 5, and 2 choices for each of the remaining 4 cells.
    In this case, there are thus 821224=28 grids.

    Finally, we look at the grids where a5 and c5 and a+c is not a multiple of 5, separating the situations where ac is a multiple of 5 and ac is not a multiple of 5.

    Case 5: a5 and c5 and a+c is a not multiple of 5 and ac is a multiple of 5

    There are 8 choices for a.
    There are then 2 choices for c: either c with the same remainder as a when divided by 5.
    There are 2 choices for each of a and c and a+c since none of a, c and a+c is a multiple of 5. aa+cc5ca Since ac is a multiple of 5, then a+c and c+a are both multiples of 5.
    To see this, note that c=5c or c=10c or c=15c, and so a+c is equal to one of 5+ac or 10+ac or 15+ac which are all multiples of 5 since ac is.
    This means that each of the empty side cells must be filled with 5.
    Finally, there are 2 choices for the bottom entry (since a+c is not a multiple of 5).
    In this case, there are 8223122=28 grids.

    Case 6: a5 and c5 and a+c is a not multiple of 5 and ac is not a multiple of 5

    There are 8 choices for a.
    There are then 4 choices for c (not 5, not either choice that makes a+c a multiple of 5, not either choice that makes ac a multiple of 5).
    There 2 choices for each of a, c, and a+c.
    There are also 2 choices for each of the 3 remaining entries in the grid since the two entries in each of the first column, third column and third row do not add to a multiple of 5.
    In this case, there are 842323=211 grids.

    Combining all of the cases, the number of possible ways to complete the grid is N=1+28+28+28+28+211=1+428+211=3073 The rightmost two digits of N are 73.

    Answer: 73


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