Factorial !

n! = n·(n-1)·(n-2)·...·3·2·1
n is whole number

3! = 3·2·1 = 6
5! = 5·4·3·2·1 = 120
Calculator has a n! button
10! = 10·9·8·7·6·5·4·3·2·1 = 3,628,800
1! = 1
0! = 1 by definition
20! = 2,432,902,008,176,640,000 ≈ 2.4 quintillion
80! ≈ 10¹¹⁸ > googol

Also:
n! = n·(n-1)!
5! = 5·4!

12! / 8! = 12·11·10·9·8! / 8! [then cancel the 8!'s] = 12·11·10·9