If np4 =20np2 find n
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nP4 = 20× nP2
n! / ( n - 4)! = 20× n! /( n - 2)!
1 / (n - 4)! = 20 × 1 / ( n - 2)(n - 3) (n - 4) !
( n - 2) (n - 3) = 20
n^2 - 3n - 2n + 6 = 20
n^2 - 5n + 6 - 20 = 0
n^2 - 5n - 14 = 0
n^2 - 7n + 2n - 14 = 0
n ( n - 7 ) + 2 ( n - 7 ) = 0
( n - 7) ( n +2 ) = 0
n = 7, - 2
n = 7
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