Factorial!!

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gmat_perfect: Legendary Member; Posts: 1083; Joined: Wed Feb 24, 2010 9:38 pmFollowed by: 14 members; Thanked: 127 times

Factorial!!

by gmat_perfect » Tue Mar 16, 2010 7:49 am

Please show me the ways to solve the following problem:

n! =90*2*(n-2)!

How can I get the value of n?

We know,

n! =n(n-1)!
Then (n-2)! =?

Thanks.

Source: Beat The GMAT — Problem Solving | Tue Mar 16, 2010 7:49 am

kstv: Legendary Member; Posts: 610; Joined: Fri Jan 15, 2010 12:33 amFollowed by: 2 members; Thanked: 47 times

by kstv » Tue Mar 16, 2010 8:30 am

it can be n! = n (n-1) (n-2)!

so n(n-1) = 90*2
then solve for nÂ²-n-180=0
it appears the roots are not real no
maybe the Q was 45*2
in which case n =10

gmat_perfect: Legendary Member; Posts: 1083; Joined: Wed Feb 24, 2010 9:38 pmFollowed by: 14 members; Thanked: 127 times

by gmat_perfect » Tue Mar 16, 2010 9:15 am

thanks.
I got it.

mj41: Senior | Next Rank: 100 Posts; Posts: 56; Joined: Wed Feb 17, 2010 3:26 am

by mj41 » Sun Mar 21, 2010 8:56 pm

I got to the wadratic equation but dont know how you got to n=10

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