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Joined: 15 May 2012
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Re: m01-q37 [#permalink] New post 17 Dec 2012, 16:15
when 69/36*100 is calculated, we should consider the digits after decimal, or else there are chances of choosing the wrong answer.
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Re: m01-q37 [#permalink] New post 20 Sep 2013, 04:18
pmal04 wrote:
Hi millhouse,
Why is it (new-old)/old ? it should have been new/old. Thanks

the questions asks for "growth" which is the same as "increase" so you only take the % difference.
btw (new-old)/old is the same as [(new/old) - 1]

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Re: m01-q37 [#permalink] New post 25 Sep 2013, 00:06
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pmal04 wrote:
There are two concentric circles with radii 10 and 8. If the radius of the outer circle is increased by 10% and the radius of the inner circle decreased by 50%, by what percent does the area between the circles grow?

(A) 140%
(B) 141%
(C) 190%
(D) 192%
(E) 292%

[Reveal] Spoiler: OA

Source: GMAT Club Tests - hardest GMAT questions

I don't agree with OA, Can anybody please explain?
When it says 'grow', we need to take the difference between new and old.
However, the OE simply does new/old instead of (new-old)/old.

Before: Area between two circles = A1 - A2 = Pi(10^2 - 8^2) = (10-8)(10+8)Pi = 36Pi
After: Area between two circles = A'1 - A'2 = Pi(11^2 - 4^2) = (11-4)(11+4)Pi = 105Pi

Percentage increase = [105 - 36]/36 * 100%
= 69/36* 100%
= [36 + 33]/36 * 100%
= 100% + 33/36*100%
= 100% + 11/12*100%
= 100% + 92%
= 192%

9/10 = 90%;
10/11 = 91%;
11/12 = 92%

This rule helps you pick a correct answer between C and D quickly.

Hope it helps.

Please +1 KUDO if my post helps. Thank you.

"Designing cars consumes you; it has a hold on your spirit which is incredibly powerful. It's not something you can do part time, you have do it with all your heart and soul or you're going to get it wrong."

Chris Bangle - Former BMW Chief of Design.

Re: m01-q37   [#permalink] 25 Sep 2013, 00:06

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