A test consists of 20 questions, and students are asked to only answer 15 of them. In how many different ways can they choose 15 questions?

N = 20, r = 15;

n C r = (n * (n - 1) (n - 2) ... (n - r + 1)) / r! =

20 C 15 = (20 * 19 * 18 * 17 ... 8 * 7 * 6) / (15 * 14 * 13 ... * 5 * 4 * 3 * 2 * 1) =

= 15,504

Answer:

They can choose 15 questions in 15,504 ways.