nP2 = n! / (n - 2)! = [ n·(n-1)·(n - 2)·(n - 3)·...·1 ] / [ (n - 2)·(n - 3)·...·1 ] = n·(n - 1)
Since nP2 = 30 ---> n·(n - 1) = 30
n2 - n = 30
n2 - n - 30 = 0
(n - 6)(n + 5) = 0
Either: n = 6 or n = -5
Since n = -5 is impossible (by definition), n = 6.