Question

Mr. Jefferson has 20 students in his class. He wants to send 4 of these students to pick up books for the class.

How many combinations of 4 students can he choose?

A.
5

B.
80

C.
4845

D.
116,280

Answer 1

this uses a formula for allocations where the order does not matter
\[which is A = \frac{ n! }{(n-m)! }\]
so 20! / 16! = 17 * 18 * 19 * 20 = 116280

Answer 2

Thanks!

Answer 3

I think the number of combination is calculated like this : \[C _{20}^{4}=\frac{ n! }{ (n-r)!*r! }=\frac{ 20! }{ 16!*4! }=4845\]