2013 AMC 10A Problem 21

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21.

A group of 1212 pirates agree to divide a treasure chest of gold coins among themselves as follows. The kthk^{\text{th}} 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 12th12^{\text{th}} pirate receive?

720720

12961296

17281728

19251925

38503850

Answer: D
Concepts:divisibilityprime factorizationwork backwards
Difficulty rating: 2300
Solution:

Work backward. If nn coins remain for the 1212th pirate, then before pirate kk took a share, the chest had 1212k\frac{12}{12-k} times as many coins as it had afterward.

Therefore the initial number of coins is n121111!n\cdot\frac{12^{11}}{11!}.

Since 121111!=214375711\frac{12^{11}}{11!}=\frac{2^{14}3^7}{5\cdot7\cdot11}, the smallest nn that makes the initial number an integer is 52711=19255^2\cdot7\cdot11=1925.

This value is attainable. The number of coins present just before pirate kk takes a share is 2143711!(12k)!12k1. 2^{14}3^7\cdot \frac{11!}{(12-k)!\,12^{k-1}}. For k=1,2,,12k=1,2,\ldots,12, all these amounts are integers. Each pirate's share is the difference between two consecutive remaining amounts, so every share is an integer as well.

Thus, the 1212th pirate receives 19251925 coins, and D is the correct answer.

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