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Re: GRE Math Challenge #26-Pump A can empty a pool in 8 minutes [#permalink]
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Hi Brent,
I did the same calculations that you did, but at the end of the problem I subtracted 2 minutes from 4.57 minutes because during those two minutes the machines were not working together. I was confused by the wording of the question since it asked for the time it took for the machines to empty the pool together, so I thought the answer would be 2.57 minutes. Will the questions on the GRE be phrased like this as well?
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Re: GRE Math Challenge #26-Pump A can empty a pool in 8 minutes [#permalink]
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AmyK wrote:
Hi Brent,
I did the same calculations that you did, but at the end of the problem I subtracted 2 minutes from 4.57 minutes because during those two minutes the machines were not working together. I was confused by the wording of the question since it asked for the time it took for the machines to empty the pool together, so I thought the answer would be 2.57 minutes. Will the questions on the GRE be phrased like this as well?
Amy


I have a feeling, the test-makers would add some extra information to prevent ambiguity.
Perhaps something like...

Pump A can empty a pool in 8 minutes. Pump B can empty a pool in 6 minutes. At 1:00pm Pump A starts pumping the pool. At 1:02pm, pump B joins in. At what time with the pool be empty?
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Re: GRE Math Challenge #26-Pump A can empty a pool in 8 minutes [#permalink]
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sandy wrote:
Pump A can empty a pool in 8 minutes.Pump B can empty a pool in 6 minutes.Pump A pumps the pool for 2 minutes before pump B joins in.How may minutes will it take to empty the pool together?


can you check the answer, since the stem says empty the pool together, I thought the answer is 18/7?
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Re: GRE Math Challenge #26-Pump A can empty a pool in 8 minutes [#permalink]
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Yh I thought so too. Carcass what do you think?
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Re: GRE Math Challenge #26-Pump A can empty a pool in 8 minutes [#permalink]
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combined rate of A+B: 7x/24 means that work done by A and B together in 1 min = 7x/24 where total work = x

Hence work left after 2 min work of A = x = (x/4) = 3x/4

Now, MT/W = constant
1 / (7x/24) = time / (3x/4)

time = 18/7

Fixed
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