Ball Problems

Man, I sure expected to post more often than once a week when I started this stupid thing. Eh, whatever, who reads this anyway? Anyway, time to introduce a new problem. I originally learned this problem somewhere randomly on the intertubes.

Consider you have a bag with n white balls and m black balls in it. You randomly pull balls out of the bag, one at a time, until you have drawn all the white balls. On average, how many black balls will be left when you stop?

Yeah, OK, thats all I got. Solution to follow later.