Calculate the combinations for C(n,r) = n! / ( r! (n - r)! ). For 0 <= r <= n.
C(n,r) = n! / ( r! (n - r)! )
The number of ways a sample of r elements can be obtained from n distinguishable objects where order does not count and repetitions are not allowed.  "The number of ways of picking r unordered outcomes from n possibilities." 
Also referred to as r-combination or "n choose r". In some resources the notation uses k instead of r so you may see these referred to as k-combination or "n choose k".
 Zwillinger, Daniel (Editor-in-Chief). CRC Standard Mathematical Tables and Formulae, 31st Edition New York, NY: CRC Press, p. 206, 2003.
For more information on combinations and binomial coefficients please see http://mathworld.wolfram.com/Combination.html