UPSC NDA Math Model Paper 1

Since the function from A to B has to be one-to-one, therefore every element of A must be mapped to exactly one element of B without repetition.
Since A = {a, b, c, d} has 4 elements and B = {1, 2, 3, 4, 5, 6} has 6 elements in it,
The total number of one-to-one functions possible is:
= (6 possibilities for mapping a) × (5 possibilities for mapping b) × (4 possibilities for mapping c) × (3 possibilities for mapping d)
= 360.