Question

Find n if: P(n,4)=20P(n,2)​

Answer 1

n(n-1)(n-2)(n-3) = 20(n)(n-1)

(n-2)(n-3) = 20

On solving above Quadratic equation we get,

n = 7 (valid) or n = -2 (invalid)

Therefore, n = 7