Brothers

A certain logic goes like this:

one is a boy. The other can be boy or girl, equally likely, so 50% (¹/₂).

There’s a certain common sense to this answer, which is what makes it being wrong so annoying.

Let’s use G for girls and B for boy.

There are four possibilities: GG, GB, BG, BB

We know it;s not GG, so that leaves three: GB, BG, BB

Of these three, only one is BB so the probability is ¹/₃ (one third).

Without the extra information you might have guessed 25%, but it isn’t.

Well done if you got that.