OG13 - DS - Q71 - how is the formula derived? - The Beat The GMAT Forum

OG13 - DS - Q71 - how is the formula derived?

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topperdoggle: Junior | Next Rank: 30 Posts; Posts: 25; Joined: Sun Aug 07, 2011 2:52 pm; Thanked: 2 times

by topperdoggle » Sat May 11, 2013 4:35 am

Answer states "If n is the number of friends... the amount contributed to the campaign was $500(n2 + n)...

I couldn't get going with this question at all - how is this formula derived?

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Source: Beat The GMAT — Data Sufficiency | Sat May 11, 2013 4:35 am

Atekihcan: Master | Next Rank: 500 Posts; Posts: 149; Joined: Wed May 01, 2013 10:37 pmFollowed by: 9 members; Thanked: 54 times

by Atekihcan » Sat May 11, 2013 7:01 am

Mary directly persuaded n of her friends to donate $500 each.
So, direct donation = $500n

Now, each of these n friends persuaded n more people to donate $500 each.
So, total people donated = nÂ² and total indirect donation from these people = $500nÂ²

So, total donation = $500n + $500nÂ² = $500(nÂ² + n)

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