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At a constant Rate of flow, it takes 20 minutes to fill a swimming poo

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Ryerson (Ted Rogers) Thread Master
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At a constant Rate of flow, it takes 20 minutes to fill a swimming poo [#permalink] New post 23 Dec 2015, 12:12
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A
B
C
D
E

Difficulty:

  5% (low)

Question Stats:

92% (01:44) correct 8% (01:23) wrong based on 50 sessions
At a constant Rate of flow, it takes 20 minutes to fill a swimming pool if a large hose is used and 30 minutes if a small hose is Used. At these constant rates, how many minutes will it take to fill the pool when both hoses are used simultaneously?


A. 10
B. 12
C. 15
D. 25
E. 50
[Reveal] Spoiler: OA

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Re: At a constant Rate of flow, it takes 20 minutes to fill a swimming poo [#permalink] New post 24 Dec 2015, 02:42
can be done in a few ways.

The std way

time taken to fill the pool by Large Hose =20 minutes =L or 1/L=1/20 similarly 1/S=1/30
simultaneously it will take 1/L+1/S=1/20+1/30=5/60=12 minutes

efficiency way (not necessarily the efficient way)

let l and s denote efficiency of large and small hose
efficiency = 100/time take to fill the pool=100/30=3.33% for smaller hose
similarly 100/20=5% for the larger hose
working together 8.33% per minute
time taken = 100/8.33 ===12.0048 ~ 12 minutes
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Re: At a constant Rate of flow, it takes 20 minutes to fill a swimming poo [#permalink] New post 26 Dec 2015, 09:10
Expert's post
Hi All,

This is a standard Work Formula question (2 or more 'entities' that work on a task together). When there are just 2 entities and there are no 'twists' to the question, we can use the Work Formula to get to the correct answer.

Work = (A)(B)/(A+B) where A and B are the individual times needed to complete the task.

We're told that two hoses take 20 minutes and 30 minutes, respectively, to fill a pool. We're asked how long it takes the two hoses, working together, to fill the pool.

(20)(30)/(20+30) = 600/50 = 12 minutes to fill the pool.

Final Answer:
[Reveal] Spoiler:
B


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Re: At a constant Rate of flow, it takes 20 minutes to fill a swimming poo [#permalink] New post 12 Jan 2016, 18:51
EMPOWERgmatRichC wrote:
Hi All,

This is a standard Work Formula question (2 or more 'entities' that work on a task together). When there are just 2 entities and there are no 'twists' to the question, we can use the Work Formula to get to the correct answer.

Work = (A)(B)/(A+B) where A and B are the individual times needed to complete the task.

We're told that two hoses take 20 minutes and 30 minutes, respectively, to fill a pool. We're asked how long it takes the two hoses, working together, to fill the pool.

(20)(30)/(20+30) = 600/50 = 12 minutes to fill the pool.

Final Answer:
[Reveal] Spoiler:
B


GMAT assassins aren't born, they're made,
Rich


But how do we know when we have to convert minutes to hours? What is an example of a more twisted problem?
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Re: At a constant Rate of flow, it takes 20 minutes to fill a swimming poo [#permalink] New post 12 Jan 2016, 19:19
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Expert's post
HI sagnik242,

The simple answer to your question is that we have to follow whatever 'instructions' we're given in the prompt. Here, we're given the two rates (time to fill the pool) in terms of 'minutes' and we're asked for an answer in 'minutes.' As such, there are no 'twists' or extra steps (and converting minutes to hours would like make all the work take longer).

If you're looking for more-complex Work questions, then you can search for them in the Quant Forums here.

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Re: At a constant Rate of flow, it takes 20 minutes to fill a swimming poo   [#permalink] 12 Jan 2016, 19:19
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At a constant Rate of flow, it takes 20 minutes to fill a swimming poo

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