what is 5!
5! is 5 factorial
It means 1*2*3*4*5
$${\mathtt{5}}{!} = {\mathtt{120}}$$
5! = 5 * 4!
4! = 4 * 3!
3! = 3 * 2!
2! = 2 * 1!
1! = 1 * 0!
0! = 1
we have: 5! = 5 * 4 * 3 * 2 * 1 * 1 = 5 * 4 * 3 * 2 * 1
