A combination is an arrangement in which order is NOT important. The notation for combinations is C(n,r) which is the number of combinations of "n" things selected "r" at a time. It is equal to the number of permutations of "n" items taken "r" at a time divided by "r" factorial. This would be P(n,r)/r! in mathematical notation.

Example: If five cards are selected from a group of nine cards, how many combinations of five cards would there be?

The number of possible combinations would be: P(9,5)/5! = (9*8*7*6*5)/(5*4*3*2*1) = 126 possible combinations.