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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% 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 (newold)/old.



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Re: m01q37 [#permalink]
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18 May 2009, 15:22
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The official answer does indeed take new/old but at the end it subtracts 1. This gives the same answer that would result had they calculated (newold)/old. Compare your answers and youll see that theyre the same. Or... (newold)/old = new/old  old/old = new/old  1. Hope that helps. millhouse pmal04 wrote: 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 (newold)/old.
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?
(C) 2008 GMAT Club  m01#37



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Re: m01q37 [#permalink]
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19 May 2009, 06:20
Hi millhouse, Why is it (newold)/old ? it should have been new/old. Thanks



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20 May 2009, 13:45
I dont know how to explain this other than by example... If 100 is increased to 120 how much did it grow by? ans: 20% i.e. (newold)/old = (120100)/100 = 20% new/old would yield 120% which is wrong. pmal04 wrote: Hi millhouse, Why is it (newold)/old ? it should have been new/old. Thanks



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Re: m01q37 [#permalink]
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26 Sep 2009, 16:42
Well the answer should be 292% ... 12136/36 .. but the OA says 192% . Am bummed.
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Re: m01q37 [#permalink]
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27 Sep 2009, 06:56
Hi, Doesn't the quoted message answer your question? If you use the formula \(\frac{new}{old}\), you have to subtract 1 to get the right answer. Hope it helps. millhouse wrote: I dont know how to explain this other than by example...
If 100 is increased to 120 how much did it grow by? ans: 20% i.e. (newold)/old = (120100)/100 = 20%
new/old would yield 120% which is wrong.
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Re: m01q37 [#permalink]
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10 Nov 2009, 09:01
The options for this question 190 and 192, are quite close. With quick calculations I came to (23/12)*100=(23/3)*25 Here is where I did the mistake and used (23/3)=7.6 (instead of 7.67 or 7.7) with the result of 190! But I feel these kind of roundoffs work in actual exams?



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Re: m01q37 [#permalink]
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11 Jan 2010, 11:29
I was also confused by getting 191...with something. I think that on the real test the question would include a phrase like "approximately" or "rounded to the nearest integer"



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Re: m01q37 [#permalink]
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17 Sep 2010, 06:23
The answer is the value of 69/36 which should be near to 200%. From the answer choices we have two options closer to 200. If you check the values of 36*190% = 68.4 (rounded to 68) and 36*192% = 69.12 (rounded to 69), 192% is the correct answer.
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17 Sep 2010, 06:50
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You don't need to calculate the area. Its just a ratio, so all you have to do is 11*114*4(10*108*8)/(10064) So, its 192%



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17 Sep 2010, 15:37
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well the correct answer is D 192 % THE explanation is as follows.....
The given radius of the concetic circle is 10 cm and 8 cm respectively so the corresponding area (πr2 ) is 100π and 64π... and the differnc betn area is (100π644π=36π) now outer radius increase by 10 % means the new outer radius is 11 cm and area in 121π inner radius decrease by 50 % means new inner radius is 4 cm and area is 16π differnce between the area is (121π  16π = 105π) percentage growth in area = (newold/old) x 100 % = (105π36π)/36π x 100% =191.77 % approx 192 %



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Re: two concentric circles with radii 10 and 8 [#permalink]
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24 Oct 2010, 18:58
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ichha148 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% Source: GMAT Club Tests  hardest GMAT questions I am getting 292. Please provide details along with your answers as well. Initially the area between the circles = 100 pie  64 pie = 36 pie new radii of outer circle = 1.1*10 = 11 new radii of inner circle = 8/2 = 4 area between them = (12116) pie = 105pie difference = 10536 = 69pie % = 69/36 * 100 = 23/12 * 100 = slightly less than 200 thus D. I think you are not dividing the difference by previous value.
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Re: two concentric circles with radii 10 and 8 [#permalink]
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24 Oct 2010, 19:06
initially the area difference 36*pi
after change, the radius of outer circle becomes 11 and that of inner circle becomes 4 area difference 105pi..
So difference increases by 105pi  36pi = 69 hence percent increase = 69/36 * 100 = 192 apprx . Hence D



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14 Aug 2011, 22:55
Area between the circles originally = 22/7 (100  64) = 22/7 (36) Area between the circles finally = 22/7 (121  16) = 22/7 (105) Percentage increase of the area between the circles = (10536)/36 * 100 = 192% (D)
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Re: m01q37 [#permalink]
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21 Sep 2011, 09:03
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Percent change problem: You can ignore pi because it cancels out of all equations.
Initial concentric circles have a difference in area of: Area 1  Area 2 = 8^2  100^2 = 36
Apply changes to radii and calculate new areas: Outer circle: 10*1.10 = 11 therefore, A=121 Inner circle: 8*.5= 4 therefore, A=16 Calculate difference in areas: 1216= 105
Then set up like any percent change problem: Change in areas= 10536 = 69 X = percent change Therefore, 69/36 = X/100 36X = 6900 X=192%



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Re: m01q37 [#permalink]
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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 YX, just as an example.
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Re: m01q37 [#permalink]
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16 Dec 2011, 17:04
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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: m01q37 [#permalink]
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21 Sep 2012, 07:31
% change = {( Area,beforeArea,after)/(Area,before) } x 100 Area,before= pi* ( Rad of bigger circle ^2 Rad of smaller circle^2)= pi (10^28^2)=pi(10064)=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,beforeArea,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: m01q37 [#permalink]
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21 Sep 2012, 08:07
In regards to the confusion about the new/old vs (newold)/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 (newold), 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..... B.
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Re: m01q37 [#permalink]
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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! B.
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