A town library is considering loaning video games, and surveyed its membership to ask their 4 favorite PlayStation 3 Games from among the following six: Gran Turismo, Call of Duty 4, Metal Gear Solid 4, Little Big Planet, Grand Theft Auto 5, & Final Fantasy 13. How many different possible sets of four favorites are there? According to the textbook, the answer should be 15, but I'm a bit lost. What are the steps I need to take or the formula I need to use in order to get this answer?