what is 5!

5!   is 5 factorial

It means 1*2*3*4*5   

$${\mathtt{5}}{!} = {\mathtt{120}}$$

what is 5!

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