
It is currently Fri Mar 23, 2018 9:05 am

View unanswered posts  View active topics

Page 1 of 1

[ 2 posts ] 

Author 
Message 
questioner

Post subject: GMAT Rate Posted: Sun Jul 28, 2013 1:40 pm 

Joined: Sun May 30, 2010 3:15 am Posts: 424

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?
A. 1/6 B. 1/3 C. 2/3 D. 3/4 E. 4/3
(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


Top 


Gennadiy

Post subject: Re: GMAT Rate Posted: Sun Jul 28, 2013 2:38 pm 

Joined: Sun May 30, 2010 2:23 am Posts: 498

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.


Top 



Page 1 of 1

[ 2 posts ] 

Who is online 
Users browsing this forum: No registered users and 1 guest 

You cannot post new topics in this forum You cannot reply to topics in this forum You cannot edit your posts in this forum You cannot delete your posts in this forum You cannot post attachments in this forum

