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Two water pumps, working simultaneously at their respective

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Joined: 26 Feb 2016
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Re: Two water pumps, working simultaneously at their respective [#permalink]

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New post 10 Jun 2017, 13:23
I prefer using numbers for rate problems such as these.
So, her goes my method of solving the question :

Since both pumps take exactly 4 hours to fill the tank, assume the tank has 200 units of water
Now, in this case, both the pumps fill 50 units/hour.
As it is given that one of the pumps fills at 1.5 times the other,
Pump A(slower pump) fills at 20 units/hour and Pump B(faster pump - 1.5*rate of slower pump) fills at 30 units/hour

Had the faster pump alone worked, it would have filled the tank in
200(Total capacity of tank)/30(Rate of filling the tank) or 20/3 hours(Option E)
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Two water pumps, working simultaneously at their respective [#permalink]

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New post 21 Jul 2017, 09:47
Bunuel wrote:
Skag55 wrote:
I did,
A+B = 4
A = 1.5B

4-B = 1.5B => B = 8/5

Why is this wrong?


We are told that "two water pumps, working simultaneously at their respective constant rates, took exactly four hours to fill a certain swimming pool".

If one pump needs A hours to fill the pool (rate=1/A) and another need B hours to fill the same pool (rate=1/B), then 1/A + 1/B = 1/4.

Solving 1/A + 1/B = 1/4 and A = 1.5B gives A=10 and B=20/3.

Check the links provided here: http://gmatclub.com/forum/two-water-pum ... l#p1245761 for more.


how can A = 1.5B be substituted here, as A stands for total time one pump needs to fill the pool and B stands for total time another pump needs to fill the pool. the ratio speaks about rates, so 1/A = 3/2B needs to be solved...
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Re: Two water pumps, working simultaneously at their respective [#permalink]

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New post 07 Aug 2017, 08:36
Its always good to assume total work in such problems. The aim should be to find per hour work of each pump

Since total time is given to be 4 hours, we will assume total work to be a multiple of 4.
Lets assume total work=40 units
Per hour work of two pumps=40/4 = 10 units
Ratio of rate(per hour work) of two pumps = 1.5/1 or 3/2
So out of a total of 10 units filled per hour, faster pump fills 6 units and slower pump fills 4 units.

If faster pump works alone
Total work= 40 units
Rate = 6 units/ hour
Time = 40/6 = 20/3

Hence E.
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Re: Two water pumps, working simultaneously at their respective [#permalink]

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New post 07 Aug 2017, 11:06
lets assume 2 pumps capacity 2.5 (as one is 1.5 more productive than another)
then amount of work produced in 4 hours will be:
2.5 * 4 = 10
keeping the same amount of work and taking into account that only one pump will be working with the productivity rate of 1.5 how many hours is needed:
1.5*x = 10
x = 100/15 = 20/3 hours
Re: Two water pumps, working simultaneously at their respective   [#permalink] 07 Aug 2017, 11:06

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