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For a certain cylinder, the diameter equals the height. [#permalink]
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24 Mar 2014, 16:20
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60% (02:12) correct 40% (01:40) wrong based on 194 sessions
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For a certain cylinder, the diameter equals the height. If every length in this cylinder is decreased by 60%, then to the nearest integer, by what percent does the volume decrease? (A) 22% (B) 40% (C) 60% (D) 84% (E) 94 % This is how I am doing it but getting the incorrect answer. Can someone please help? Case 1: Assume diameter = 10 then radius = 5 and height will be 10 Volume = pi*r^2*h = 250pi (1) Decrease height by 60 % New height = 4 New volume = pi*25*4 = 100pi(2) Volume decrease in percent = Old  new / old *100 250 pi  100 pi/250pi * 100 = 60 % which is not correct.
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Re: For a certain cylinder, the diameter equals the height. [#permalink]
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24 Mar 2014, 19:47
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Answer = E = 94% Area of Cube = \(\pi r^2 h\) Given that 2r = h So the Area of cube = \(2 \pi r^3\) ................. (1) All dimension are decreased by 60%, so new dimension is \(\frac{40r}{100}= \frac{2r}{5}\) Now the area of cube = \(2 \pi (\frac{2r}{5})^3\) = \(2 \pi r^3 . (\frac{2}{5})^3\) ......................... (2) Reduction in the area = (1)  (2) =\(2 \pi r^3 (1  \frac{8}{125})\) = \(2 \pi r^3 (\frac{117}{125})\) ................ (3) Comparing equation (1) & (3) ; finding the percentage reduction = \(\frac{\frac{117}{125} . 2 \pi r^3 . 100}{2 \pi . r^3}\) \(= 93.6 =\approx{94}\) Answer = E
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For a certain cylinder, the diameter equals the height. [#permalink]
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24 Mar 2014, 20:00
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enigma123 wrote: For a certain cylinder, the diameter equals the height. If every length in this cylinder is decreased by 60%, then to the nearest integer, by what percent does the volume decrease?
(A) 22%
(B) 40%
(C) 60%
(D) 84%
(E) 94 %
This is how I am doing it but getting the incorrect answer. Can someone please help?
Case 1: Assume diameter = 10 then radius = 5 and height will be 10
Volume = pi*r^2*h = 250pi (1)
Decrease height by 60 %
New height = 4
New volume = pi*25*4 = 100pi(2)
Volume decrease in percent = Old  new / old *100
250 pi  100 pi/250pi * 100 = 60 % which is not correct. In this case, the equation (2) is incorrect Problem says "If every length in this cylinder is decreased by 60%"; means height & radius both are decreased by 60%So the new volume would be \(2 . \pi . (\frac{40}{100})^3 . 5^3\) New volume = \(16 \pi\) ............... (2) Reduction = \(250 \pi  16 \pi = 234 \pi\) Percentage reduction = \(234 * \frac{100}{250} = 93.6\) \(\approx{94}\)
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Last edited by PareshGmat on 08 Aug 2014, 03:28, edited 1 time in total.



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Re: For a certain cylinder, the diameter equals the height. [#permalink]
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25 Mar 2014, 02:40
enigma123 wrote: For a certain cylinder, the diameter equals the height. If every length in this cylinder is decreased by 60%, then to the nearest integer, by what percent does the volume decrease? (A) 22% (B) 40% (C) 60% (D) 84% (E) 94 % This is how I am doing it but getting the incorrect answer. Can someone please help?
Case 1: Assume diameter = 10 then radius = 5 and height will be 10
Volume = pi*r^2*h = 250pi (1)
Decrease height by 60 %
New height = 4
New volume = pi*25*4 = 100pi(2)
Volume decrease in percent = Old  new / old *100
250 pi  100 pi/250pi * 100 = 60 % which is not correct. The area of the cylinder is \(\pi{r^2}h\). Since given that the diameter equals the height, then 2r=h and the area becomes \(2\pi{r^3}\). Now, if we decrease r by 60% it becomes 0.4r and thus the new volume becomes \(0.4^3*(2\pi{r^3})=0.064*(2\pi{r^3})\). Therefore the volume decreased by approximately 10.06=0.94=94%. Answer: E.
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Re: For a certain cylinder, the diameter equals the height. [#permalink]
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25 Mar 2014, 02:57
A shorter approach : Such questions can be done without even using the full formula for the volume of cylinder. We just need to consider the exponent of the variables in the formula <In this case '\(r\)' and '\(h\)'> The exponent of '\(r\)' is \(2\) and the exponent of '\(h\)' is \(1\) in the formula. And since both the quantities decrease by the same percentage, the '\(new r\)' is .4r and '\(new h\)' is .4h. So the new volume will be = \((.4r)^2 * (.4h)\) <we can neglect \pi since \pi is there in both the cases  new volume as well as original volume> New volume(V') = (.4)^3 * r^2 *h = .064 r^2 * h => V' = .064 V
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Re: For a certain cylinder, the diameter equals the height. [#permalink]
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18 Jan 2015, 04:02
after doing the calculations the volume decrease comes to 96.8 percent closet integer given is 94. hence E.



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Re: For a certain cylinder, the diameter equals the height. [#permalink]
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18 Jan 2015, 06:54
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enigma123 wrote: For a certain cylinder, the diameter equals the height. If every length in this cylinder is decreased by 60%, then to the nearest integer, by what percent does the volume decrease?
(A) 22%
(B) 40%
(C) 60%
(D) 84%
(E) 94 %
This is how I am doing it but getting the incorrect answer. Can someone please help?
Case 1: Assume diameter = 10 then radius = 5 and height will be 10
Volume = pi*r^2*h = 250pi (1)
Decrease height by 60 %
New height = 4
New volume = pi*25*4 = 100pi(2)
Volume decrease in percent = Old  new / old *100
250 pi  100 pi/250pi * 100 = 60 % which is not correct. Given, all the lengths of the cylinder are reduced by 60% Hence all the lengths are now 40% of actual! Original Volume (V)= (pi)(r*r)(h) New Volume (V1) = (pi)(0.4r*0.4r)(0.4h) = (0.064)(pi)(r*r)(h) = 0.064V Thus new Volume is 6.4% of original Volume This implies the volume has reduced by 93.6% which after rounding off becomes 94% Hence (E)



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Re: For a certain cylinder, the diameter equals the height. [#permalink]
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18 Jan 2015, 13:43
Hi All, Since this question does not include any specific dimensions for the cylinder, we can TEST VALUES. We're told that the Diameter = Height IF.... Diameter = Height = 10 Volume = pi(R^2)(H) V = pi(5^2)(10) V = 250pi We're then told that both measurements are reduced by 60%... Diameter = Height = 4 New Volume = pi(2^2)(4) New Volume = 16pi The question asks us for the approximate percentage change in the Volume, so we need the Percentage Change Formula: (New  Old)/Old = (16  250)/250 = 234/250 From here, we don't really need to calculate; we can use the answer choices to our advantage and do "comparison" math instead... 10% of 250 = 25 234 is "16 away" from 250, so it's less than 10% away. As such, the percentage decrease in the volume of the cylinder is greater than 90%. Final Answer: GMAT assassins aren't born, they're made, Rich
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Re: For a certain cylinder, the diameter equals the height. [#permalink]
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