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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]
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23 Dec 2015, 13:12
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89% (00:42) correct 11% (00:56) wrong based on 296 sessions
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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
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Re: At a constant Rate of flow, it takes 20 minutes to fill a swimming poo [#permalink]
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24 Dec 2015, 03:42
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]
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26 Dec 2015, 10:10
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: GMAT assassins aren't born, they're made, Rich
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Re: At a constant Rate of flow, it takes 20 minutes to fill a swimming poo [#permalink]
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12 Jan 2016, 19:51
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: 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]
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12 Jan 2016, 20:19
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 morecomplex Work questions, then you can search for them in the Quant Forums here. GMAT assassins aren't born, they're made, Rich
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Re: At a constant Rate of flow, it takes 20 minutes to fill a swimming poo [#permalink]
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16 Mar 2017, 00:25
RSOHAL wrote: 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 In order to solve this question we need to know the combined work rate of the small hose and large hose. Small Hose= 1 job / 20 mins Large Hose= 1 job/ 30 mins We must set the denominators equal and add the two fractions 3 jobs/ 60 mins + 2 jobs/ 60 mins = 5 jobs / 60 mins Therefore 1 job / 12 mins 1 job is done in 12 minutes at this combined work rate.



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At a constant Rate of flow, it takes 20 minutes to fill a swimming poo [#permalink]
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06 May 2017, 17:48
I had approached the question by picking numbers, but not sure if this is correct  can someone please verify if I what did below is correct or not?
Since in the question, they don't tell us the quantity to fill the pool with each hose, I just chose 60 liters as an estimate, therefore:
For large hose: 60L /20 min = 3L in 1 min For small hose: 60L/30 min = 2L in 1 min
Combined rates: 3L + 2L = 5L in 1 min together, so if we wanted to find how much it would take to fill up to the original 60L, that would mean you multiplied 5L by 12, therefore, 60 L would fill up in 12 min with the two hoses. Does this make sense or is the approach of picking numbers incorrect?
Thanks!



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Re: At a constant Rate of flow, it takes 20 minutes to fill a swimming poo [#permalink]
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07 May 2017, 11:38
Hi infinitemac,
YES  your approach (TESTing VALUES) works just fine on this question. Since the prompt tells us nothing about the actual rates of the two hoses nor about the volume of the swimming pool, you can choose whatever values you like  as long as they properly account for the times that it takes the two hoses to fill the pool individually.
GMAT assassins aren't born, they're made, Rich



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Re: At a constant Rate of flow, it takes 20 minutes to fill a swimming poo [#permalink]
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14 Aug 2017, 00:23
20*30/(20+30)= 12 mins
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Re: At a constant Rate of flow, it takes 20 minutes to fill a swimming poo
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