2015 AMC 10B Problem 15

Below is the professionally curated solution for Problem 15 of the 2015 AMC 10B, from LIVE by Po-Shen Loh. You can also try the full timed exam, view all 2015 AMC 10B solutions, or check the answer key.

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Concepts:Diophantine Equationmodular arithmetic

Difficulty rating: 1420

15.

The town of Hamlet has 33 people for each horse, 44 sheep for each cow, and 33 ducks for each person. Which of the following could not possibly be the total number of people, horses, sheep, cows, and ducks in Hamlet?

41 41

47 47

59 59

61 61

66 66

Solution:

If there are hh horses and cc cows, then there are 3h3h people, 9h9h ducks, and 4c4c sheep. The total is therefore 13h+5c13h+5c.

The listed values except 4747 can be written in that form: 41=132+53,59=133+54,61=132+57,66=132+58.41=13\cdot2+5\cdot3,\quad 59=13\cdot3+5\cdot4,\quad 61=13\cdot2+5\cdot7,\quad 66=13\cdot2+5\cdot8. For 4747, subtracting 0,13,26,390,13,26,39 leaves 47,34,21,847,34,21,8, none of which is divisible by 55.

Thus, the correct answer is B.

Problem 15 in Other Years