A baby is added to a hospital nursery. Before the baby was added there were two boys in the nursery and an uncounted number of girls. After the new baby is added a baby is selected at random among all the babies. The selected baby is a boy.

What is the probability that the added baby was a girl?

(In reply to I have not read Charlie's post yet by FatBoy)

Consider this: you have a box with one red ball and one white ball. Someone adds a ball of one or the other color, but you can't see which. You try a ball at random and see that it's red. Doesn't that increase the probability, in your estimation, that the ball that was added had been red?

Each time you pull out a red ball in your random trials, it makes it more likely that the ball placed in there had been in fact red.

Posted by Charlie on 2003-10-08 15:46:02