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by bblast » Mon Jun 06, 2011 1:26 am
https://www.beatthegmat.com/files/marydo ... on_435.jpg

[spoiler]I was able to solve this by why is the equation n(n+1) instead of simply n^2 [/spoiler]

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Source: — Data Sufficiency |
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by galaxian » Mon Jun 06, 2011 1:35 am
Coz' we need to add the n peoples too (Mary's n direct friends) along with n^2 people (Friends of friends)..hence
n^2 + n = n(n+1)
When in doubt, always try to put nos like take n = 2 & you will find total 6 people [(n)(n+1)] & not 4 will be there in our case.
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by cans » Mon Jun 06, 2011 1:55 am
n friends donated 500. then n*n people donated 500 each.
thus total money = 500(n + n*n)
a) we know n*500 = (1/16)*500(n + n*n)
16=1+n
n=15
Sufficient
b) 500*(n+n*n) = 120,000
n(n+1)=240
n=15
Sufficient.
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