Tuesday, September 2, 2014

Summing up to fifteen, two player game puzzle

[Heard this somewhere, but don't recollect the source]

Alice and Bob (no, this is not a crypto puzzle) are playing a game.

They have 9 coins, each with a unique integer from 1 to 9 written (i.e. all the numbers 1,2,...,9 appear).

They take turns picking coins, picking exactly one coin at their turn, and add the picked coin to their respective collection.

If at any time, any player has three unique coins (from their collection) which sum to 15, that player wins.

Here is an example game:

Alice picks 1
Bob picks 9
Alice picks 8
Bob picks 6
Alice picks 2

At this point, Alice's collection has 1, 2, 8 and Bob's collection has 6,9, and Bob hasn't won because we need three coins to sum to 15.

Alice always goes first.

Does anyone have a winning strategy?

[This has a pretty neat solution.]

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