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An empty pool being filled with water at a constant rate [#permalink]

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20 Dec 2012, 05:53

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An empty pool being filled with water at a constant rate takes 8 hours to fill to 3/5 of its capacity. How much more time will it take to finish filling the pool?

(A) 5 hr 30 min (B) 5 hr 20 min (C) 4 hr 48 min (D) 3 hr 12 min (E) 2 hr 40 min

An empty pool being filled with water at a constant rate takes 8 hours to fill to 3/5 of its capacity. How much more time will it take to finish filling the pool?

(A) 5 hr 30 min (B) 5 hr 20 min (C) 4 hr 48 min (D) 3 hr 12 min (E) 2 hr 40 min

As pool is filled to 3/5 of its capacity then 2/5 of its capacity is left to fill.

Since it takes 8 hours to fill 3/5 of the pool, then to fill 2/5 of the pool it will take 8/(3/5)*2/5 = 16/3 hours = 5 hours 20 minutes (because if t is the time needed to fill the pool then t*3/5=8 --> t=8*5/3 hours --> to fill 2/5 of the pool 8*5/3*2/5=16/3 hours will be needed).

Or plug values: take the capacity of the pool to be 5 liters --> 3/5 of the pool or 3 liters is filled in 8 hours, which gives the rate of 3/8 liters per hour --> remaining 2 liters will require: time = job/rate = 2/(3/8) = 16/3 hours = 5 hours 20 minutes.

Re: An empty pool being filled with water at a constant rate [#permalink]

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26 Apr 2014, 19:12

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There are a couple of different ways to approach the math in this question. It looks like you started to set up a ratio, but didn't complete the work. Here's one way to go about it:

Since it takes 8 hours to fill 3/5 of the pool and X hours to fill 2/5 of the pool.....

8/X = (3/5)/(2/5)

Since both fractions are "over 5", we can multiply those 5s out (by multiplying the numerator and denominator by 5)....

If it takes 8 hours to fill 3/5 of the pool, then it takes 8/3 hours to fill 1/5 of the pool, and it thus takes 16/3 hours to fill 2/5 of the pool, which is what we need to do.
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If you are looking for online GMAT math tutoring, or if you are interested in buying my advanced Quant books and problem sets, please contact me at ianstewartgmat at gmail.com

An empty pool being filled with water at a constant rate takes 8 hours to fill to 3/5 of its capacity. How much more time will it take to finish filling the pool?

(3/5) An empty pool being filled with water at a constant rate takes 8 hours to fill to 3/5 of its capacity. How much more time will it take to finish filling the pool? (3/5) of a pool/ 8 hours = 3/40 (the rate)

(3 pools/40 hours) = (2/5* pool)/ x hours Cross multiply 3x = (2/5) 40 3x = (2/5) (8) (5) 3x = 16 x = 16/3 or 5 1/3 1/3 of an hour = 20 minutes

* The pool is 3/5 full so 2/5 remains.

(A) 5 hr 30 min (B) 5 hr 20 min (C) 4 hr 48 min (D) 3 hr 12 min (E) 2 hr 40 min
_________________

Re: An empty pool being filled with water at a constant rate [#permalink]

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17 May 2016, 05:59

We know, R * T = Q , suppose the Q=100ltr. then, R * 8 = 3/5 of 100 ==> 60/8 ==> 15/2 ltr per hour.

now, remaning 40ltr(which is 100 - 60 or 2/5 of 100) R * t = Q or, 15/2 * t = 40 or, t = 40 * 2/15 t = 16/3 = 5.33 = 5 hour + 1/3 and 1/3 of 60 = 20. so, 5 hour 20 mins.

Re: An empty pool being filled with water at a constant rate [#permalink]

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17 May 2016, 17:01

Walkabout wrote:

An empty pool being filled with water at a constant rate takes 8 hours to fill to 3/5 of its capacity. How much more time will it take to finish filling the pool?

(A) 5 hr 30 min (B) 5 hr 20 min (C) 4 hr 48 min (D) 3 hr 12 min (E) 2 hr 40 min

Solution:

To solve we can setup a proportion. The proportion will read:

A time of 8 hours is to filling up 3/5 of the pool is the same as a time of x number of hours is to filling up (the remaining) 2/5 of the pool. Setting this up mathematically we have:

8/(3/5) = x/(2/5)

8/(3/5) = x/(2/5)

40/3= 5x/2

Cross multiplying, we get:

80 = 15x

16 = 3x

16/3 = x

5 1/3 hours = x

5 hours and 20 minutes = x

Answer: B
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Jeffrey Miller Jeffrey Miller Head of GMAT Instruction

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Re: An empty pool being filled with water at a constant rate
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17 May 2016, 17:01

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