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m01-q37

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Re: m01-q37 [#permalink] New post 22 Sep 2011, 09:58
You wanna know why we divide the difference by the "old" at the end? When we say "X have changed to Y, now by what percent has X changed?" We actually mean by what percent of X ITSELF has X changed. Clear? So the denominator must again be X. The procedure is exactly like when they ask you Z is what percent of X. Then, you divide Z by X. Here Z is represented by Y-X, just as an example.
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Re: m01-q37 [#permalink] New post 16 Dec 2011, 17:04
Unless there is some kind of trick I am not aware of, the answer will have to be devised by long division - all the way to the 4th number in order to accurately identify the solution. Not very typical for GMAT questions, since GMAC claims they are not testing our long division skills - rather our ability to find short exact ways to an answer.
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Re: m01-q37 [#permalink] New post 14 Jan 2012, 23:18
IMO 192 % is the correct answer
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Re: m01-q37 [#permalink] New post 21 Sep 2012, 07:31
% change = {( Area,before-Area,after)/(Area,before) } x 100

Area,before= pi* ( Rad of bigger circle ^2- Rad of smaller circle^2)= pi (10^2-8^2)=pi(100-64)=36 pi

The situation later:
10% increase in bigger radius will make 10 units of radius to 11 units of radius
50% decrease in smaller radius will make 8 units of radius to 4 units of radius

Area , after=pi*(11^2 - 4^2) = 105 pi

putting the values of both areas in main formula;

% change = { ( Area,before-Area,after)/(Area,before)} x 100

={ (105 pi - 36 pi )/ 36 pi } x 100
={ 69 pi/ 36 pi} x 100
=191.6666666666667 ( do a guess work don't calculate pin point values like this in real exam ; 69/36 appx equals to less than 2 so in ACs 192 is nearest to 200)
192 appx.

D wins
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Re: m01-q37 [#permalink] New post 21 Sep 2012, 08:07
In regards to the confusion about the new/old vs (new-old)/old question above, I think the clue is in the language the question uses:
'grow' means 'change' (specifically in a positive direction). In mathmatical terms we deal with change as (new-old), so the corrent answer should you the same logic.

For the new/old varient to work, the question would have to say something like 'the new area is what percent of the old area'.

My thoughts anyways.....

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Re: m01-q37 [#permalink] New post 21 Sep 2012, 08:10
Quote:
Unless there is some kind of trick I am not aware of, the answer will have to be devised by long division - all the way to the 4th number in order to accurately identify the solution. Not very typical for GMAT questions, since GMAC claims they are not testing our long division skills - rather our ability to find short exact ways to an answer.

Not necessarily, the GMAT definately uses approximations as a short cut (e.g 7.99854/3.98 is roughly 2). In this case, I think the little trick the question is using is to get us to identify that 23/12 is v. close to 24/12 = 2. There is only one choice that is just a touch under 2 so voila!

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Re: m01-q37 [#permalink] New post 17 Dec 2012, 17:15
when 69/36*100 is calculated, we should consider the digits after decimal, or else there are chances of choosing the wrong answer.
Re: m01-q37   [#permalink] 17 Dec 2012, 17:15
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