Suppose I flip two coins, without letting you see the outcome, and I tell you that at least one of the coins came up heads, what is the probability that the other coin is also heads?
For two coins, there are four possible outcomes: HH, HT, TH, TT, since we know that at least one was a head, we can eliminate TT. Of the remaining three possibilities, only 1 allows the second head: HH.
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Today's brain teaser courtesy of Braingle.com.