LogFAQs > #876637143

Someone has probably posted the answer already, but I can't be assed to read through the entire topic.

The correct answer is 1/3, which seems counter-intuitive, but it's true based on the information we are given (namely that one of the children is a girl - the problem is that we don't know "which one").

Consider that the kids had to be born in a specific order (yes, even if they were twins). In this case, there's four possible combinations of birth order for two kids:

Boy, then boy.
Boy, then girl.
Girl, then boy.
Girl, then girl.

Because we know one of the children is a girl, the first combination is impossible, leaving just the other three. Two of those feature a boy and a girl, one of those features two girls. Thus, the odds of having two girls is one-in-three (although if you provided more information - like specifying whether the girl was the older or younger sibling - you could change the odds to an even 50%).

Sorry, ~80% of PotD, you're wrong.
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