Question:
If (n + 1)! = 90 [(n − 1)!], find n.
Solution:
(n + 1)! = 90 [(n − 1)!]
$\Rightarrow(n+1) \times(n) \times(n-1) !=90[(n-1) !]$
$\Rightarrow(n+1) \times(n)=90$
$\Rightarrow(n+1) \times(n)=10 \times 9$
On comparing, we get:
n = 9
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