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Source: Official Guide for the GMAT 13th Ed. Problem Solving; #103 Official Guide for the GMAT 2015 14th Ed. Problem Solving; #103

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The water from one outlet, flowing at

The water from one outlet, flowing at a constant rate, can fill a swimming pool in 9 hours. The water from a second outlet, flowing at a constant rate, can fill the same fool in 5 hours. If both outlets are used at the same time, approximately what is the number of hours required to fill the pool?

3 Explanations

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outlet·······W············T
A·············1 pool······9
B·············1 pool······5
A+B·········1 pool······T

Since the work done is equal in all the cases, we can use:

T(A+B) = [T(A) * T(B)] / [T(A) + T(B)]

T(A+B) = [9 * 5] / [9 + 5]

T(A+B) = [9 * 5] / [14]

T(A+B) = [45] / [14] = 3.21

Jan 26, 2015 • Comment

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Andrew Flores

5/45 + 9/45 = 14/45. Not 13/45.

Jan 10, 2015 • Comment

Jonathan , Magoosh Tutor

Hi! You're definitely right about that! I'll make a note to our content team so we can get this fixed. Thanks for pointing that out! :)

Jan 11, 2015 • Reply

Sean Gregory

What he said :)

Mar 11, 2015 • Reply

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Mike McGarry, Magoosh Tutor

Dec 28, 2013 • Comment

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