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Can't understand this problem.....

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Can't understand this problem.....

by yumi2012 » Tue Sep 03, 2013 6:19 pm
A closed cylindrical tank contains 36Ï€ cubic feet of water and is filled to half its capacity. When the tank is placed upright on its circular base on level ground, the height of the water in the tank is 4 feet. When the tank is placed on its side on level ground, what is the height, in feet, of the surface of the water above the ground?

(A) 2
(B) 3
(C) 4
(D) 6
(E) 9

Official guide question
-------

According to the answer guide, the answer is 3... which surprised me.. So the water in the tank was actually 18pai? And the 36 pai was just the capacity of the tank?
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by ganeshrkamath » Tue Sep 03, 2013 7:58 pm
yumi2012 wrote:A closed cylindrical tank contains 36Ï€ cubic feet of water and is filled to half its capacity. When the tank is placed upright on its circular base on level ground, the height of the water in the tank is 4 feet. When the tank is placed on its side on level ground, what is the height, in feet, of the surface of the water above the ground?

(A) 2
(B) 3
(C) 4
(D) 6
(E) 9

Official guide question
-------

According to the answer guide, the answer is 3... which surprised me.. So the water in the tank was actually 18pai? And the 36 pai was just the capacity of the tank?
The question basically asks you to find the radius of the cylinder.
Volume = 36Ï€*2 = 72Ï€
Volume = π (r^2) h
h = 4*2 = 8

72π = 8 (r^2) π
r^2 = 9
r = 3

Choose B

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by [email protected] » Wed Sep 04, 2013 11:11 pm
Hi Yumi2012,

As you continue to practice, you'll find that many GMAT questions come with little hints (sometime Number Properties, sometimes patterns) that you can use to your advantage.

In this case, the prompt tells you that the cylinder is "half full"; this means that regardless of what "side" you lay the cylinder on, the water will go "half-way" up. When you lay the cylinder on its side, "half-way" would be the radius.

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