Three bags are marked $10, $15, and $20. One bag contains two $5 dollar bills, one contains a $5 and a $10 bill, and one contains two $10 bills. You are told that no bag contains the amount of money that is marked on its exterior. You are allowed to select a bag and extract a bill. How many times must you do this before before you can guarantee that you know the contents of all three bags? What is your strategy?

A bag contains three cards, one blue on both sides, one green on both sides, and one with one blue side and one with one green side. You pull one card from the bag and place it on the table. The side showing is blue. What is the probability that the side not showing is also blue?