Quickest way to solve this ACT math question

Test Preparation
1

If (n+1)!/(n-1)!=20 , then n!=?

A.6
B. 10
C. 12
D.24
E.120

Thanks in advance!

2

Simplifying, (n+1) * n = 20. Obviously 54 = 20, so n = 4. Or n^2 + n = 20, n^2 + n - 20= 0. So n = 4 or -5 by the quadratic formula, factoring, or completing the square. Discard the negative solution. 4! = 4321 = 24

3

n(n+1)=20 -> positive value for n=4-> 4!=24