Originally Posted by

**whitehat** Hi, I'm new to this site. Sorry I'm jumping in with a question as the first post!

There was a question in my textbook that goes like this:

"A family has 2 kids. Given that at least one of them is a boy, what is the probability that both of them are boys?"

My idea was this:

Initially the kids could satisfy one of three cases:

boy/boy, boy/girl, girl/girl Mr F says: You've missed an 'outcome': girl/boy. See below.

But now that we are given that at least one of them is a boy, we can eliminate the case girl/girl. Out of the two remaining cases, one is favorable, so the probability should be 1/2.

But the answer as given in the book is 1/3. What am I doing wrong?

Thanks in advance.