Practice Problems for  the Lander Math Contest
Module 6, Problems (16-18):

16) Find the binomial coefficients a) 10C6 b) 13C7.

17) Prove that a)
nCn-2=n(n-1)/2 and b) nCn-3=n(n-1)(n-2)/6

18) Show that
nCk= nCn-k.