Number of different permutations (order counts) when n different items are available, but
only r of them are selected without replacement
Example: A fan of Lady Antebellum music plans to make a custom CD with 12 of their 27 songs to listen to while
the run. Since they need additional motivation at the end on their run they have decided to put their most favorite
songs last (in effect ranking them in reverse order). How many different combinations of 12 songs are possible? Is it
practical to make a different CD for each possible combination? N = 27 R = 12