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You are given three coins: one has heads on both faces, the second has tails on both faces, and the third has a head on one face and a tail on the other. You choose a coin at random and toss it, and it comes up heads. The probability that the other face is tails is

1. 1/4
2. 1/3
3. 1/2
4. 2/3

Correct answer: 1/3

Solution

Given that the toss shows heads, the possible head outcomes are from the double-headed coin and the fair coin, with the double-headed coin contributing twice as many head outcomes as the fair coin. Among the head outcomes, only one corresponds to a coin whose other face is tails, so the probability is 1/3.

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