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

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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: — Data Sufficiency |
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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 = n² and total indirect donation from these people = $500n²

So, total donation = $500n + $500n² = $500(n² + n)
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