How factorial of 0 is 1

Dear Kushal

we know that "number of permutation of n distinct thing taken r at a time is

 ⁿP_r=n!/(n-r)!=n(n-1)(n-2)(n-3)............(n-(r-1))

put r=n

ⁿP_n=n(n-1)(n-2)(n-3)...........3.2.1

n!/(n-n)! =n!

n!/0!=n!

0!=1