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|Author:||questioner [ Sun Jul 28, 2013 1:40 pm ]|
|Post subject:||GMAT Rate|
A group of 3 small pumps and 1 large pump is filling a tank. Each of the 3 small pumps works at 2/3 of the rate of the large pump. If all 4 pumps work at the same time, they will fill the tank in what fraction of the time that it would have taken the large pump had it operated alone?
(B) Since each of the small pumps does 2/3 as much work as the large pump, the 3 small pumps will work at a rate of 3 × 2/3 = 2 times as fast as the large pump.
So the large pump and the 3 small pumps combined can pump 3 times as much as the large pump could by itself.
Therefore, they will finish the job in 1/3 the time it would take the large pump to do the job by itself.
The correct answer is choice (B).
need more explanation .. very confused
|Author:||Gennadiy [ Sun Jul 28, 2013 2:38 pm ]|
|Post subject:||Re: GMAT Rate|
Let's go the other way.
If the large pump works at a rate of x ft³/minute, for example, then one of the small pumps works at (2/3)x ft³/minute and all the three small pumps pour 2x ft³/minute.
The small pumps and the large pump combined have a rate of 2x + x = 3x ft³/minute. In other words they work 3 times "faster" than the large pump alone, which means they will fill the same volume 3 times faster.
For example, if the large pump fills the tank in 1 hour, then the large and the three small pumps together will fill it in 1/3 hour.
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