This is a GRE math problem

Question

Set A has n different numbers, Set A has exactly 6 subsets that have 2 numbers. What's the value of n?

Answer 1

_nC₂ = 6

n! / ((n - r)!r!) = 6

n! / ((n - 2)!2!)) = 6

n! / (n - 2)! = 12

n(n - 1) = 12

n² - n - 12 = 0

n = 4