A textbook search committee is considering 18 books for possible adoption. The committee has decided to select 4 of the 18 for further consideration. In how many ways can it do so?

This is a combination problem, since order does not matter. Specifically, it is an 18 - choose - 4.

₁₈C₄ = 18! : (4! (18-4) !

= 18! : (4! (14!)) The 14! cancels out in the numerator/denominator

= 18 x 17 x 16 x 15 : (4 x 3 x 2)

= 3,060