Question

n!(n+1)!
--------------=20
(n-1)!n!

Answer 1

n! . (n + 1) . n . (n - 1)!
----------------------------- = 20
(n - 1) ! . n!

(n + 1) . n = 20
n^2 + n = 20
n (n + 1) = 20
n = 20 ou n = 19