Combinatorics
(n,r)
C (n,2)=10
Find n
\(\frac{n!}{2!*(n-2)!}=10\\ \frac{n!}{(n-2)!}=20\\ n(n-1)=20\\ n^2-n-20=0\\ (n-5)(n+4)=0\\ n=5 \qquad \text{Because n cannot be negative}\)
