Bunuel wrote:
If 5a + 7b = k, where a and b are positive integers, what is the largest possible value of k for which exactly one pair of integers (a, b) makes the equation true?
A. 35
B. 48
C. 69
D. 70
E. 74
Kudos for a correct solution.
MANHATTAN GMAT OFFICIAL SOLUTION:Work Backwards from the AnswersThe largest answer choice (and the largest possible value for k) is 74, so the largest possible value for b is 10. (If b were 11, then 7b would equal 77, which is too large.)
What about for a? Use similar reasoning: the largest possible value for a must be 14 because 5(14) = 70, which is less than 74.
The possibilities for b, then, are 1, 2, 3…9. The possibilities for a are 1, 2, 3…14.
List out the possibilities for 5a and 7b:
Because the question asks for the largest possible value of k, start with the largest answer choice. Do any of these pairs add to 74? Something from the 7b column would need to be added to a multiple of 5 (that is, a number that ends in either 5 or 0). Check the units digits. One possibility is that 7b + something with a unit’s digit of 5 = something with a unit’s digit of 4, so 7b would need to have a unit’s digit of 9. For example, 49 + 25 = 74. Is (E) the answer?
Hang on. The question asks for the largest possible number for which exactly one pair of integers (a, b) will work. We’ve just found one pair, but is there another? Keep checking. No other values for 7b end in 9, but we could also have 7b + a unit’s digit of 0 = a unit’s digit of 4, in which case the 7b would have to have a unit’s digit of 4. Are there any values for 7b that end in 4? Yes: 14. 14 + 60 = 74. For 74, then, at least two pairs (a, b) exist. Answer (E) is not correct.
What about answer (D), 70? Here, if the 5a value ends in 5, then 7b would also have to end in 5. Are there any values of 7b that end in 5? Yes, one: 35. 35 + 35 = 70, so one possible (a, b) pair does exist. Is there another? No other values of 7b end in 5. We also have to check what would happen if 5a ends in 0, though—in that case, 7b would also have to end in 0. The only value for 7b that ends in 0 is 70, but that would leave 5a = 0, and that’s not permitted (the problem says that a is a positive integer).
The only possible (a, b) pair, then, that makes k = 70 is (7, 5).
The correct answer is (D).It is also possible to solve the problem with theory, but the solution is pretty tricky.
Note that the least common multiple of 5 and 7 is 5 x 7 = 35. Therefore, if a solution may be found where a > 7, then there will definitely be at least two solutions to the equation, since it will be possible to “trade” seven 5’s for five 7’s. For instance, if a = 8, then 5(8) + 7b = k, meaning that k would be the sum of eight 5’s and some number of 7’s. In that case, we could trade seven of the eight 5’s for five 7’s, so that k can also be written as 5(1) + 7(b + 5).
For the same reason, if b > 5, there will also be at least two solutions to the equation.
The maximum possible values for a and b (when told that there can’t be more than one solution pair for a and b) are thus 7 and 5, respectively. Can any other pairs of values for a and b also produce k = 70? Check as we did above; the answer is no. The only pair is a = 7 and b = 5, so k = 70 is the answer.
The correct answer is (D).Attachment:
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