C(20,18) gives us our n and r
n = 20
r = 18
[tex] \frac{n}{(n - r) \times r} [/tex]
With factorials
You can equate n! to 20 * 19 * 18! on top
2! on bottom with 18!
Which you can remove 18! because of division giving you
20 * 19 / 2!
This changes to 20 * 19 / 2 = 190
As for C(6,6), these always equate to 1
