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# The volume of a cylinder is pi * r^2 * h, where r is its

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Manager
Joined: 20 Mar 2005
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Location: Colombia, South America
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The volume of a cylinder is pi * r^2 * h, where r is its [#permalink]  13 Nov 2005, 17:31
The volume of a cylinder is pi * r^2 * h, where r is its radius and h is its height.
we have two different cylinders, both cylinders have the same height = 8 cms
but cylinder 1 has a diameter of 4 and cylinder 2 has a diameter of 8.
we melt both cylinders and create a sphere, the volume of the sphere is 4/3*pi*r^3, what is the diameter of the sphere?

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Senior Manager
Joined: 02 Oct 2005
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I believe we should use the surface area instead of the volume. Here is how I approached the problem. Correct me if I am wrong.

Surface area of cylinder = 2 * pi * r^2 + 2 * pi * r * h
Surface area of sphere = 4 * pi * r^2

Surface area of sphere = SA of C1 + SA of C2

4 * pi * r^2 = 136 pi

r^2 = 34

r = sqrt(34)
Diameter = 2 * sqrt(34)

Btw, do we have OA?
SVP
Joined: 16 Oct 2003
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I am getting r^3 = 120
r =~5
d = 10
Director
Joined: 27 Jun 2005
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volume of First cy + volume of Second cy = volume of sphere

r^3=120
yup

4<r<5....

so diametere is close to 10

what is OA ...?
Manager
Joined: 20 Mar 2005
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Location: Colombia, South America
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I made this question up, as I got something similar in the real test, not as hard but similar, I agree with Jonny's answer.

Add both volumes and then find r from the sphere, and then multiply by two to find d
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