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Two pipes, A and B, empty into a reservoir. Pipe A can fill the reserv

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Bunuel
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Two pipes, A and B, empty into a reservoir. Pipe A can fill the reserv [#permalink] New post   16 Oct 2017, 10:28
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Two pipes, A and B, empty into a reservoir. Pipe A can fill the reservoir in 30 minutes by itself. How long will it take for pipe A and pipe B together to fill up the reservoir?

(1) By itself, pipe B can fill the reservoir in 20 minutes.
(2) Pipe B has a larger cross-sectional area than pipe A.
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vaibhavanant
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Re: Two pipes, A and B, empty into a reservoir. Pipe A can fill the reserv [#permalink] New post   16 Oct 2017, 11:18
Two pipes, A and B, empty into a reservoir. Pipe A can fill the reservoir in 30 minutes by itself. How long will it take for pipe A and pipe B together to fill up the reservoir?

(1) By itself, pipe B can fill the reservoir in 20 minutes. --- Since from the question we have that the work done by Pipe A = 1/30. We only needed work done by B hence sufficient.

(2) Pipe B has a larger cross-sectional area than pipe A- Does not talk about the work done by B. Might be possible that the water flow is too slow or too fast. Hence not Sufficient

IMO Answer : A

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vaibhavanant
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Re: Two pipes, A and B, empty into a reservoir. Pipe A can fill the reserv [#permalink] New post   18 Oct 2017, 01:56
Hi @Bunuel
Please share the OA.

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Bunuel
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Re: Two pipes, A and B, empty into a reservoir. Pipe A can fill the reserv [#permalink] New post   18 Oct 2017, 02:03
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vaibhavanant wrote:
Hi Bunuel
Please share the OA.

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The OA will be automatically revealed on Monday 23rd of October 2017 10:28:56 AM Pacific Time Zone
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Plunkster82
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Re: Two pipes, A and B, empty into a reservoir. Pipe A can fill the reserv [#permalink] New post   23 Oct 2017, 08:32
I got A. My first instinct was, this is easy it's obviously A. However, when I do that I almost always miss the problem and do something wildly stupid. Any suggestions on avoiding easy mistakes would always be helpful.

We know pipe A can do it in 30 mins. Pipe B can do it in 20 mins.
If we were to solve, the formula here is ((30*20)/(30+20))=600/50=12 minutes.
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Re: Two pipes, A and B, empty into a reservoir. Pipe A can fill the reserv [#permalink] New post   23 Oct 2017, 12:38
Can someone please provide a mathematical solution to this problem.
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Re: Two pipes, A and B, empty into a reservoir. Pipe A can fill the reserv [#permalink] New post   23 Oct 2017, 12:43
kartzcool see the answer above you. That is the mathematical expression. There are a few ways to do it, but I usually use the formula AB/(A+B).
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Re: Two pipes, A and B, empty into a reservoir. Pipe A can fill the reserv [#permalink]
23 Oct 2017, 12:43
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