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Simple problem-Water pump

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Simple problem-Water pump

by Ashetty » Wed Sep 21, 2011 12:54 pm
I got answer right.Simple problem-But someone please explain me how to calculate in simple way?..so that I can apply the logic in similar problems??Thanks a ton!!

A small water pump would take two hours to fill empty tank, A large pump would take 1/2 hr to fill same tank, How many hours it will take both pumps working together at their respective constant rate to fill the empty tank if they begin pumping at same time??
A.1/4
B.1/3
C.2/5
D.5/4
E.3/2

ANS:[spoiler]C.2/5[/spoiler]

Thanks!!
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by shankar.ashwin » Wed Sep 21, 2011 1:08 pm
Consider both work for 1 hr;

Then the big pump can fill 2 tanks and the small pump can fill 1/2 a tank, so in total they can fill 5/2 tanks per hour.

If they work for H hours together at a combined rate of 5/2, then time taken to fill one tank is

5/2 * H = 1 (1 for filling the full tank)

So; H = 2/5
Ashetty wrote:I got answer right.Simple problem-But someone please explain me how to calculate in simple way?..so that I can apply the logic in similar problems??Thanks a ton!!

A small water pump would take two hours to fill empty tank, A large pump would take 1/2 hr to fill same tank, How many hours it will take both pumps working together at their respective constant rate to fill the empty tank if they begin pumping at same time??
A.1/4
B.1/3
C.2/5
D.5/4
E.3/2

ANS:[spoiler]C.2/5[/spoiler]

Thanks!!
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by tpr-becky » Wed Sep 21, 2011 2:09 pm
A consistent way to approach rate problems involves these big concepts:
- each situation gets it's own rate formula
- if you work together, add the rates
- If it says "a---" (a pool, a job, a tank, a wall etc.) pick a number to represent the units instead of using 1 and working with fractions:

thus a small pump takes 2 hours to fill an empty tank

Pick 6 as the number of gallons in the tank and set up the formula (Rs)(2) = 6 so Rs = 3

a large tank can fill the same tank in 1/2 hour - Rl(.5) = 6 so Rl= 12

if they work together how long to fill the tank:

(12 + 3)(T)= 6 or t = 6/15 = 2/5
Becky
Master GMAT Instructor
The Princeton Review
Irvine, CA
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