Water Pumps

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Ashetty: Senior | Next Rank: 100 Posts; Posts: 49; Joined: Sun May 08, 2011 4:33 pm

by Ashetty » Mon Sep 26, 2011 2:26 pm

Two water pump working simultaneously at their respective constant rate took exactly 4 hours to fill certain swimming pool.If the constant rate of one pump was 1.5 times the constant rate of the other, How many hours would it taken the faster pump to fill the pool if it had worked alone?
A.5
B.16/3
C.11/2
D.6
E.20/3

Ans:[spoiler]E.20/3[/spoiler]

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Source: Beat The GMAT — Problem Solving | Mon Sep 26, 2011 2:26 pm

tpr-becky: GMAT Instructor; Posts: 509; Joined: Wed Apr 21, 2010 1:08 pmFollowed by: 85 members; Location: Irvine, CA; Thanked: 199 times; GMAT Score:750

by tpr-becky » Mon Sep 26, 2011 2:42 pm

In these problems with "A pool" (a job, a wall etc...) it is a good idea to name a number of units - here we want the numbers to work well with 4 so name the gallons of the pool to be 100. Teh formula for rate is D = R(t) adn wehn you work together you add the rates of the two machines.

thus 100 = (A+B)(4) which means that rate of A plus rate of B = 25. We also know that
A = 1.5B (doesn't really matter which one is which but you need to remember). Thus 1.5B + B = 25

or 2.5B = 25 Thus B = 25/2.5 which means B = 10. If B is 10 then A = 15. A is the faster pump so we need to know how long it would take A to fill the pool. Use the formula again:

100 = (T)(15) so the time is 100/15 - reduce by 5 and you get 20/3 which is E.

Becky
Master GMAT Instructor
The Princeton Review
Irvine, CA

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