OG 13, Data Sufficiency Q71.

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vishrutsolanki: Newbie | Next Rank: 10 Posts; Posts: 7; Joined: Tue Jan 24, 2012 8:29 am; Location: Mumbai, India

OG 13, Data Sufficiency Q71.

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

Before you achieve you must believe.

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Source: Beat The GMAT — Problem Solving | Tue Mar 19, 2013 6:21 am

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Brent@GMATPrepNow: GMAT Instructor; Posts: 16207; Joined: Mon Dec 08, 2008 6:26 pm; Location: Vancouver, BC; Thanked: 5254 times; Followed by:1268 members; GMAT Score:770

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

Brent Hanneson - Creator of GMATPrepNow.com