What Is a Paradox?

By jamie

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2. The Boy or Girl Paradox

A couple has two children, and we know that one of the children is a boy. This must mean that there’s a 50/50 chance of the other child being a girl, right? Not quite. There are four possible combinations: (BB), (GG), (BG) where the boy is the oldest, and (GB) where the girl is the oldest.

We know that one child is a boy, which eliminates (GG), but there are still 3 options remaining, meaning the probability of the other child being a girl to be 2/3. The question can also be asked in different ways, with wording that can mean the probability has to be 1/2, 1/3, or 2/3.

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