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2013 AMC 12A Problem 17

Problem 17 of 25IntermediateNumber TheoryArithmetic

A group of 1212 pirates agree to divide a treasure chest of gold coins among themselves as follows. The kkth pirate to take a share takes k12\dfrac{k}{12} of the coins that remain in the chest. The number of coins initially in the chest is the smallest number for which this arrangement will allow each pirate to receive a positive whole number of coins. How many coins does the 1212th pirate receive?

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Solution

For 1≤k≤11,1 \le k \le 11, the number of coins before the kkth pirate takes a share is 1212−k\dfrac{12}{12 - k} times the number afterward. So if nn coins are left for the 1212th pirate, the initial count is 1211 n11!=214⋅37 n52⋅7⋅11. \dfrac{12^{11}\, n}{11!} = \dfrac{2^{14}\cdot 3^{7}\, n}{5^2\cdot 7\cdot 11}. The smallest nn making this a positive integer is 52⋅7⋅11=1925.5^2\cdot7\cdot11=1925. Before pirate k,k, the remaining count is the initial count multiplied by 11!(12−k)! 12k−1;\frac{11!}{(12-k)!\,12^{k-1}}; substituting this nn shows it is an integer for every k.k. Hence all shares are integral, and the 1212th pirate receives 19251925 coins. Thus, the correct answer is D.
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Tagged: divisibility · prime factorization · factorial

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