Step-by-step explanation:
Permutation :The number of ways to choose a sample of r elements from a set of n distinct objects where order does matter and replacements are not allowed.
Factorial : There are n! ways of arranging n distinct objects into an ordered sequence.
Considering a situation when n = r in a permutation, nPr reduces to n!, a simple factorial of n.
Proof: 3P3 = 3!
n = 3 and r = 3
But 0! = 1
3P3 = 3!