Problem 4 (12 points). Let r > 3. Write an expression (with proof) for the number of r- permutations of the multiset M = {1.a, 2.b,c,d,e, op.f}. Problem 4 (12 points). Let r > 3. Write an expression (with proof) for the number of r- permutations of the multiset M = {1.a, 2.b,c,d,e, op.f}