A brand of cereal puts one out a set of 10 different plastic animals into every box of cereal.

Little Johnnie gets a kangaroo in his first box of cereal, and likes it so much that he wants a second

kangaroo so that he has a family to play with. What is the expected number of additional cereal

boxes Johnnie must buy in order to get that second kangaroo? Assume that the animal in each box

is equally likely to be any one of the 10 different animals.