Solution:
Here, multiple of 3 (3, 6, 9, 12, 15, 18) For 4, 8, 12, 16, 20, there are 6 x 5 x 4 x 3 x 2 = 6! ways For rest numbers, there are 15! ways
Total ways
The number of functions f from {1, 2, 3, …. , 20} onto {1, 2, 3, … , 20} such that f(k) is a multiple of 3, whenever k is a multiple of 4 is
The number of functions from {1, 2, 3, .... , 20} onto {1, 2, 3, ... , 20} such that is a multiple of 3, whenever is a multiple of 4 is
