BrainteasersHeads and Tails
You have three coins in a bag. One is a regulation coin with "heads" on one side and "tails" on the other. The two other coins are counterfeit: one has heads on both sides and the other has tails on both sides.
Give the bag a good shaking, withdraw one coin at random and lay it on the table without looking at the down face.
The up face is a heads. What is the probability the down face is tails?
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