Question

http://awesomescreenshot.com/00e2duyy27

Answer 1

Do you know the formula for permutations?

Answer 2

\(\large ^np_r = \frac{n!}{(n-r)!}\)

Answer 3

btw, 5! = 5*4*3*2*1
2! = 2*1
3! = 3*2*1

"!" is called "factorial"

Answer 4

I never knew that o.o thank you