[10] 4. Suppose there are 20 different types of collectible cards and suppose that each time you buy a card, it is equally likely to be any one of the 20 types. Let X be the number of types of cards you still do not have after buying 10 random cards. Find E(X ) and Var(X). (Hint: Define indicator variables X1, X2, ... X20, where 1, if type i is not obtained Xi = 10, otherwise and remember that the number of pairs (i, j) with i 0.5