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?

I find that no matter how you slice it, The nursery is %100 full of babies after the addition, 2 boys are in the mix. Confusion is that there could be 1 or many Girls. If there were xGirls and 2Boys, we will call the last addition "a" (a is equal to "1" because only one was added to the mix then (xG-B+1=n)
Probability is: (n/xG)+1
Example if 50Girls, 2 are Boys + 1 Addition.
n=49 so 49/50 or 98%.

Posted by Bill on 2003-10-22 13:19:04