In mathematics, permutation is the different arrangement of a given number of things by taking some or all at a time.
The combination is a way of selecting items from a collection, such that the order of selection does not matter.
Let n be a positive integer. Then, factorial n, denoted n! is defined as:
n! = n(n - 1)(n - 2) ... 3.2.1.
Number of all permutations of n things, taken r at a time, is given by:
If there are n subjects of which p1 are alike of one kind; p2 are alike of another kind; p3 are alike of third kind and so on and pr are alike of rth kind, such that (p1 + p2 + ... pr) = n.
Then, number of permutations of these n objects is =