Maths Olympiad Prep

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Combinatorics Difficulty 5.7 AIME, harder Find the answer Italy

Alberto, Barbara, and Carlo are playing cards. In each hand, the winner gains 2 points, while the other two players each lose one point. Initially, everyone has 0 points. What is the probability that, after 10 hands, everyone is again at zero points?

Pick one

Solution

The answer is (A). We want to show that it is impossible for all the players, after 10 hands, to have zero points again.

Let vv be the number of hands won by the first player when everyone finds themselves again with 0 points, and pp the number of hands lost by him.

Then, by the rule given in the text on the assignment of points, we find
0=2kp 0 = 2k - p
By hypothesis we have k+p=10k + p = 10, the total number of hands played; substituting into the previous equation we find 0=2k(10k)0 = 2k - (10 - k), that is 10=3k10 = 3k.

The only solution to this equation is k=103k = \frac{10}{3}, absurd since kk, representing the number of hands won by the first player, should be an integer. The assumed situation is therefore impossible, and the probability that all players have 0 points is thus 0.

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Source: MathNet, licensed CC-BY-4.0. Statement translated into English from it; metadata (topic, difficulty) added by this project.