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OG 13, Data Sufficiency Q71.

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by vishrutsolanki » Tue Mar 19, 2013 6:21 am
Hey all !! I have a problem with the understanding of answer explanation of DS Q71 of OG 13.

The problem says that Mary persuaded n friends to donate 500$ and then each of these n friends persuaded n more people to donate 500$ each.

Now the answer explanation says that the total contribution made is $500(n^2+n).

Can some one explain how did the calculate the total contribution.
Vishrut Solanki

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by Brent@GMATPrepNow » Tue Mar 19, 2013 6:27 am
vishrutsolanki wrote:Hey all !! I have a problem with the understanding of answer explanation of DS Q71 of OG 13.

The problem says that Mary persuaded n friends to donate 500$ and then each of these n friends persuaded n more people to donate 500$ each.

Now the answer explanation says that the total contribution made is $500(n^2+n).

Can some one explain how did the calculate the total contribution.
First round: n friends donate 500 dollars.
This gives us a total of 500n dollars in this round

Second round: n friends persuade n friends each to donate
So, each of the n friends gets n more people to donate.
The total number of donors in this round = n^2
This gives us a total of 500(n^2) dollars in this round

Total donations = 500n dollars + 500(n^2) dollars
We can rewrite this as 500(n + n^2) dollars
Or even 500n(1+n) dollars

Cheers,
Brent
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by vishrutsolanki » Tue Mar 19, 2013 6:41 am
Thank you Brent. That was simpler than i thought. Had spent at least 30 mins trying to figure out the expression. Thank you one more time.
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