Here is the data. You know that your cousin has exactly two children and one of them is a girl.

What are the odds the other child is a boy?

The answer, of course, is not 50% but 67%.

Here is why. Your cousin has 2 children. The four birth order possibilities are:

1 girl -- girl
2 girl -- boy
3 boy -- girl
4 boy -- boy

Since one child is a girl, possibility 4 is ruled out.

Of the three left, two include a boy; only one has two girls. 2/3 = 67%. Q.E.D.

Got your head around that one? Let's add one more piece of data to your knowledge.

You still know that your cousin has exactly two children and one of them is a girl. But you also know that the girl's name is Myrtle. Now what are the odds?

They are no longer 67%.

Go to Solutions, to see why.

John Burgeson

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