Given: (n + 1)! = 90 [(n – 1)!]

To Find : n ?

⇒ (n + 1) × (n) × (n-1)! = 60 [(n – 1)!]

⇒ (n + 1) × (n) = 90

⇒ (n + 1) × (n) = 10 × 9

n = 9

Hence, n = 9