3. Let n 2 t 2 0. Recall that dy is the number of derangements on {1, 2, . .. , r}, and dr, is the number of permutations of {1, 2, ... ,r} with exactly s fixed points (so in particular, dro = dr. Give a combinatorial proof of each equality. a) dn,t = (7 )dn-t b) n! = Et-odn,t