One of four different prizes was randomly put into each box of a cereal. If a family decided to buy this cereal until it obtained at least one of each of the four different prizes, what is the expected number of boxes of cereal that must be purchased? (Hint: Let X = # of boxes of cereal needed to get four prizes. Think of X as the sum of four random variables. Yi = # of boxes needed to get a prize you don't already have, i = 1, 2, 3, 4.)