When there are 2 pirates: the youngest gets zero and all of the coins go to the older one.

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When there are 3 pirates: the youngest one gets 1 coin, the middle gets nothing, and the oldest gets the rest of the coins. 

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When there are 4 pirates: the second youngest gets 1 coin, the oldest gets the rest, and the other two pirates get nothing. 

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Interested in learning more?  Check out Ian Stewart's 'A Puzzle for Pirates,' published in the Mathematical Recreations section of Scientific America, May 1999.