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# Choices: 10 20 40 80 100 The only thing I thought of when I

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Intern
Joined: 14 Aug 2008
Posts: 21
Location: United States
Choices: 10 20 40 80 100 The only thing I thought of when I [#permalink]

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18 Aug 2008, 23:46
[(1001^2)-(999^2)] / [(101^2)-(99^2)]

Choices:
10
20
40
80
100

The only thing I thought of when I first saw this question in GMATPrep was to solve as quickly as possible.

I thought the answer should be 10 or 100 because the numbers look like multiples of 10... but I really have no idea how the heck to estimate.

Can anyone help?

Thanks!

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Senior Manager
Joined: 16 Jul 2008
Posts: 285

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18 Aug 2008, 23:57
It goes like this:

(1001 - 999)x(1001 + 999)/(101-99)x(101+99) = 2x2,000/2x200 = 10

Very useful rule for the GMAT:
x^2 - y^2 = (x-y)(x+y)

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Re: How to estimate?   [#permalink] 18 Aug 2008, 23:57
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# Choices: 10 20 40 80 100 The only thing I thought of when I

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