Question

find n if, 10Pn-1:11Pn-2=30:11

Answer 1

{ 10! / ( 11 - n )! } / { 11! / (13 - n)! }
=> { 10! / 11! } * { (13-n) (12-n) } = 30/11
=> 1/11 * { (13-n) (12-n) } = 30/11
=> (13-n) (12-n) = 30

NOTE: (13 - n) and ( 12 - n) are consecutive terms...
Therefore,  (13-n) (12-n) = 6 * 5 or (-6) * (-5)
=> n = 13 - 6 or 12 - (-6)
=> n = 7 or 18

Well, you can use Quadratics of you want......... Hope you're satisfied..