800score.comhttp://www.800score.com/forum/ GMAT Ratehttp://www.800score.com/forum/viewtopic.php?f=3&t=15407 Page 1 of 1

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

 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.

 Page 1 of 1 All times are UTC - 5 hours [ DST ] Powered by phpBB © 2000, 2002, 2005, 2007 phpBB Grouphttp://www.phpbb.com/