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The surface area of cylinder A is how many times the surface  [#permalink]

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Question Stats: 37% (01:35) correct 63% (01:07) wrong based on 197 sessions

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The surface area of cylinder A is how many times the surface area of cylinder B?

(1) The diameter of A is twice the diameter of B.
(2) The height of A is equal to the height of B.

This is a "what is the value of..." DS question. In this type of question, a statement will be sufficient only if it leads to a single value of the variable (or expression) you're asked about.

Remember, surface area of a cylinder = 2 π r² + 2 π r h where r is the radius of the cylinder, h is its height and π is a constant. The ratio between the surface areas of the two cylinders can be determined either by calculating the two surface areas or by identifying relationships between r and h of the two.

Stat. (1) + (2) Combined, you get the relationship between the radii as well as heights of the two cylinders. Let 2r be the radius of cylinder A and r be the radius of cylinder B as per Stat. (1), and let h be the height of both cylinders as the height of cylinder A is equal to that of cylinder B. By plugging in these in the formula of the surface area of a cylinder, you find that the surface area of cylinder A = 8 π r² + 4 π r h and that of cylinder B = 2 π r² + 2π r h. It is not possible to determine the ratio between the surface areas of cylinder A and B without knowing the specific values of r and h.

Stat.(1) + (2)->IS->E.

So I understand the math and was able to get to the point of cylinder A = 8 π r² + 4 π r h and that of cylinder B = 2 π r² + 2π r h.

But the provided solution only says that "It is not possible to determine the ratio between the surface areas of cylinder A and B without knowing the specific values of r and h."

I don't understand why. I chose C because we're given the relationship between the two diameters, in addition to H.

Could someone help me to understand?

Originally posted by anon1 on 02 Nov 2012, 18:09.
Last edited by Bunuel on 30 May 2015, 04:49, edited 1 time in total.
Edited the question.
VP  Joined: 02 Jul 2012
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Location: India
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Re: The surface area of cylinder A is how many times the surface  [#permalink]

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anon1 wrote:
The surface area of cylinder A is how many times the surface area of cylinder B?
(1) The diameter of A is twice the diameter of B.

(2) The height of A is equal to the height of B.

This is a "what is the value of..." DS question. In this type of question, a statement will be sufficient only if it leads to a single value of the variable (or expression) you're asked about.

Remember, surface area of a cylinder = 2 π r² + 2 π r h where r is the radius of the cylinder, h is its height and π is a constant. The ratio between the surface areas of the two cylinders can be determined either by calculating the two surface areas or by identifying relationships between r and h of the two.

Stat. (1) + (2) Combined, you get the relationship between the radii as well as heights of the two cylinders. Let 2r be the radius of cylinder A and r be the radius of cylinder B as per Stat. (1), and let h be the height of both cylinders as the height of cylinder A is equal to that of cylinder B. By plugging in these in the formula of the surface area of a cylinder, you find that the surface area of cylinder A = 8 π r² + 4 π r h and that of cylinder B = 2 π r² + 2π r h. It is not possible to determine the ratio between the surface areas of cylinder A and B without knowing the specific values of r and h.

Stat.(1) + (2)->IS->E.

So I understand the math and was able to get to the point of cylinder A = 8 π r² + 4 π r h and that of cylinder B = 2 π r² + 2π r h.

But the provided solution only says that "It is not possible to determine the ratio between the surface areas of cylinder A and B without knowing the specific values of r and h."

I don't understand why. I chose C because we're given the relationship between the two diameters, in addition to H.

Could someone help me to understand?

$$Surface area of A = 2*pi*2r*(2r+h)$$

$$Surface area of B = 2*pi*r*(r+h)$$

$$\frac{Area_A}{Area_B} = \frac{2*pi*2r*(2r+h)}{2*pi*r*(r+h)}$$

$$= \frac{2*(2r+h)}{(r+h)}$$

So to know the ratio, we need atleast r in terms of h. What makes a hemisphere,cylinder, cone, and cuboid different from a sphere and square is the presence of a "+" in the calculation of the total surface area.

While the given data would have been sufficient had the cylinders been open at the top and bottom, it is not sufficient in the case of normal cylinders.

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GMAT 1: 690 Q48 V35 GMAT 2: 710 Q48 V39 Re: The surface area of cylinder A is how many times the surface  [#permalink]

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Wording of Question is ambiguous... It is not clear if it is asking total surface area or just lateral/ curved surface area...could someone clarify ???

Posted from my mobile device
Math Expert V
Joined: 02 Sep 2009
Posts: 58320
Re: The surface area of cylinder A is how many times the surface  [#permalink]

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smashbiker84 wrote:
Wording of Question is ambiguous... It is not clear if it is asking total surface area or just lateral/ curved surface area...could someone clarify ???

Posted from my mobile device

It's total area. If it were otherwise it would have been explicitly mentioned.
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Re: The surface area of cylinder A is how many times the surface  [#permalink]

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Hi Bunuel,

I can´t understand why is E, with the proportion of diameter and the same height is just not Cylinder A two times Cylinder B?

Or I should analyse something else? Regards
Omar M
Intern  Joined: 02 Jun 2016
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Schools: Sloan '19 (II)
Re: The surface area of cylinder A is how many times the surface  [#permalink]

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If you find this question on the real GMAT, you should first be very wary because each statement alone is quite obviously insufficient. Even if you didn't know the surface area formula, it should be clear looking at each statement that you need both pieces of info. This is a very clear C trap question and I think the best bet would be to just try plugging in easy numbers to see both statements fall short.

Make the height equal to 1
Try radius 2 for bigger cylinder, radius 1 for smaller, then try radius 4 for bigger, 2 for smaller. Quick calculations and it becomes clear answer is E

Sent from my SM-G928T using GMAT Club Forum mobile app
Intern  B
Joined: 29 Nov 2015
Posts: 7
Re: The surface area of cylinder A is how many times the surface  [#permalink]

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LXVE wrote:
If you find this question on the real GMAT, you should first be very wary because each statement alone is quite obviously insufficient. Even if you didn't know the surface area formula, it should be clear looking at each statement that you need both pieces of info. This is a very clear C trap question and I think the best bet would be to just try plugging in easy numbers to see both statements fall short.

Make the height equal to 1
Try radius 2 for bigger cylinder, radius 1 for smaller, then try radius 4 for bigger, 2 for smaller. Quick calculations and it becomes clear answer is E

Sent from my SM-G928T using GMAT Club Forum mobile app

Thanks LXVE , Now it´s clear.
VP  P
Joined: 12 Dec 2016
Posts: 1493
Location: United States
GMAT 1: 700 Q49 V33 GPA: 3.64
The surface area of cylinder A is how many times the surface  [#permalink]

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bb, what is Curved Surface Area? I have seen 1 question that mentioned such term.
Math Expert V
Joined: 02 Sep 2009
Posts: 58320
Re: The surface area of cylinder A is how many times the surface  [#permalink]

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chesstitans wrote:
bb, what is Curved Surface Area? I have seen 1 question that mentioned such term.

You don't need to know this for the GMAT. Still if interested check here: https://www.quora.com/What-is-the-diffe ... rface-area
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Re: The surface area of cylinder A is how many times the surface  [#permalink]

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_________________ Re: The surface area of cylinder A is how many times the surface   [#permalink] 07 Jul 2019, 12:49
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