In an office n different letters are placed randomly into n addressed envelopes. Let Zn denote the number of correct mailings. For each k {1, . . . , n}, let Xk be the random variable defined by Xk = 1, if the k-th letter is placed correctly 0, otherwise . a) Find E(Xk) and V (Xk) for each k {1, . . . , n}. b) Find E(Zn) and V (Zn). c) How many correct mailings are to be expected