Solve the following problem, asked of Marilyn Vos Savant in the "Ask Marilyn" column of Parade Magazine, February 18, 1996. Say I have a wallet that contains either a $2 bill or a $20 bill (with equal likelihood), but I don’t know which one. I add a $2 bill. Later, I reach into my wallet (without looking) and remove a bill. It’s a $2 bill. There’s one bill remaining in the wallet. What are the chances that it’s a $2 bill?