ANSWER. k! = 1*2*3* . . . *(k-1)*k. Explanation. Let the sets be A and B: they have equal numbers of elements. 1st element of A can be related to any of k elements of B. 2nd element of A can be related to any of remaining (k-1) elements of B. 3rd element of A can be related to any of remaining (k-2) elements of B. . . . and so on . . . It gives the answer.
" how many are there permutations of a set of k elements ? "