Number of different combinations (order does not count) when n different items are available,
but only r of the n items are selected without replacement. (Note that rearrangements of the same items are counted
as being the same.)
Example: Winning the jackpot in the Florida Lotto requires that you choose six different numbers from 1
to 53, and your numbers must match the same six numbers that are later drawn. The order of the selected
numbers does not matter. If you buy one ticket, what is the probability of winning the jackpot? N = 53 R = 6