For this case we have that by definition:
[tex]C (n, r) = \frac {n!} {r! * (n-r)!}[/tex]
We have to:
[tex]n = 7\\r = 2[/tex]
Substituting:
[tex]C (7,2) = \frac {7!} {2! * (7-2)!}[/tex]
By definition:
[tex]7! = 7 * 6 * 5!\\2! = 2 * 1 = 2[/tex]
Substituting:
[tex]C (7,2) = \frac {7 * 6 * 5!} {2 * 5!}\\C (7,2) = \frac {7 * 6} {2}\\C (7,2) = \frac {42} {2}\\C (7,2) = 21[/tex]
Answer:
21
Option A
