Gmat if a group of 3 is to be chosen from a group of 6, how many different groups are possible?

The number of combinations of taking "r" items from a group of "n" is found by the formula:
combinations = n! / [r! * (n-r)!]
combinations = 6! / 3! * 3!
combinations = 6 * 5 * 4 * 3! / 36
combinations = 20

Source:
http://www.1728.org/combinat.htm