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

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

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New post 16 Oct 2017, 09:28
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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.
[Reveal] Spoiler: OA

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Kudos [?]: 135364 [0], given: 12691

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Re: Two pipes, A and B, empty into a reservoir. Pipe A can fill the reserv [#permalink]

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New post 16 Oct 2017, 10:18
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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Re: Two pipes, A and B, empty into a reservoir. Pipe A can fill the reserv [#permalink]

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New post 18 Oct 2017, 00:56
Hi @Bunuel
Please share the OA.

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Re: Two pipes, A and B, empty into a reservoir. Pipe A can fill the reserv [#permalink]

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New post 18 Oct 2017, 01:03

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Re: Two pipes, A and B, empty into a reservoir. Pipe A can fill the reserv [#permalink]

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New post 23 Oct 2017, 07:32
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]

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New post 23 Oct 2017, 11:38
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]

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New post 23 Oct 2017, 11:43
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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I'd love to hear any feedback or ways to improve my problem solving. I make a lot of silly mistakes. If you've had luck improving on stupid mistakes, I'd love to hear how you did it.

Also, I appreciate any kudos.

Kudos [?]: 8 [0], given: 18

Re: Two pipes, A and B, empty into a reservoir. Pipe A can fill the reserv   [#permalink] 23 Oct 2017, 11:43
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Two pipes, A and B, empty into a reservoir. Pipe A can fill the reserv

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