8
7
6
5
Correct answer is C
nC2 = 15
n!(n−3)!2! = 15
n(n−1)(n−2)!(n−2)!2 = 15
n2 - n = 30
n2 - n - 30 = 0
(n - 6)(n + 5) = 0
n = 6 or n = -5
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