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A tank has both an inlet pipe and an outlet pipe. Working
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Updated on: 21 Jan 2014, 03:21
Question Stats:
75% (02:36) correct 25% (02:51) wrong based on 356 sessions
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A tank has both an inlet pipe and an outlet pipe. Working alone, the inlet pipe can fill up the tank in 5 hours. Working alone, the outlet pipe can empty out the tank in 15 hours. If it is desired that the tank should be filled, from empty, exactly 6 hours after the inlet pipe is turned on at 9:30am, then at what time should the outlet pipe be opened ? A. 10:00am B. 10:45am C. 12:00pm D. 12:30pm E. 1:30pm
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Originally posted by guerrero25 on 20 Jan 2014, 07:24.
Last edited by guerrero25 on 21 Jan 2014, 03:21, edited 1 time in total.




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Re: A tank has both an inlet pipe and an outlet pipe. Working
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20 Jan 2014, 07:47
guerrero25 wrote: A tank has both an inlet pipe and an outlet pipe. Working alone, the inlet pipe can fill up the tank in 5 hours. Working alone, the outlet pipe can empty out the tank in 15 hours. If it is desired that the tank should be filled, from empty, exactly 6 hours after the inlet pipe is turned on at 9:30am, then at what time should the outlet pipe be opened ?
A) 10:00am
B) 10:45am
C) 12:00pm
D) 12:30pm
E) 1:30pm
I do not have the OA with me right now . In 6 hours the inlet pipe will pump in water equal to 6*1/5 = 6/5 of the pool. So, 1/5 more than necessary, which should be compensated by the outlet pipe. To pump out 1/5 of the water the outlet pipe needs 15/5 = 3 hours. Therefore, the outlet pipe should be opened at 9:30am + 6 hours  3 hours = 12:30pm. Answer: D.
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Re: A tank has both an inlet pipe and an outlet pipe. Working
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20 Jan 2014, 22:05
Bunuel wrote: guerrero25 wrote: A tank has both an inlet pipe and an outlet pipe. Working alone, the inlet pipe can fill up the tank in 5 hours. Working alone, the outlet pipe can empty out the tank in 15 hours. If it is desired that the tank should be filled, from empty, exactly 6 hours after the inlet pipe is turned on at 9:30am, then at what time should the outlet pipe be opened ?
A) 10:00am
B) 10:45am
C) 12:00pm
D) 12:30pm
E) 1:30pm
I do not have the OA with me right now . In 6 hours the inlet pipe will pump in water equal to 6*1/5 = 6/5 of the pool. S o, 1/5 more than necessary, which should be compensated by the outlet pipe. To pump out 1/5 of the water the outlet pipe needs 15/5 = 3 hours.Therefore, the outlet pipe should be opened at 9:30am + 6 hours  3 hours = 12:30pm. Answer: D. Bunuel, but to pump out the excess 1/5 of water, should we not use the combined rate (inflow+outflow)? The combined rate would be 10l outflow/hour. So time required will be 10/5 = 2 hours. Therefore, the outlet pipe should be opened at 9:30am + 6 hours  2 hours = 1:30pm. Any mistake here?



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Re: A tank has both an inlet pipe and an outlet pipe. Working
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20 Jan 2014, 22:45
Let us say that the capacity of the tank is 30 liters Inlet  6 liters per hour Outlet  2 liters per hour Let us say it should be turned on after x hours. Total time taken to fill 6 hours 6x + (4)(6x) = 30 6x + 24  4x = 30 2x = 6 x = 3 So it should be opened after 3 hours i.e. 12:30 am
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Re: A tank has both an inlet pipe and an outlet pipe. Working
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21 Jan 2014, 01:43
HKHR wrote: Bunuel wrote: guerrero25 wrote: A tank has both an inlet pipe and an outlet pipe. Working alone, the inlet pipe can fill up the tank in 5 hours. Working alone, the outlet pipe can empty out the tank in 15 hours. If it is desired that the tank should be filled, from empty, exactly 6 hours after the inlet pipe is turned on at 9:30am, then at what time should the outlet pipe be opened ?
A) 10:00am
B) 10:45am
C) 12:00pm
D) 12:30pm
E) 1:30pm
I do not have the OA with me right now . In 6 hours the inlet pipe will pump in water equal to 6*1/5 = 6/5 of the pool. S o, 1/5 more than necessary, which should be compensated by the outlet pipe. To pump out 1/5 of the water the outlet pipe needs 15/5 = 3 hours.Therefore, the outlet pipe should be opened at 9:30am + 6 hours  3 hours = 12:30pm. Answer: D. Bunuel, but to pump out the excess 1/5 of water, should we not use the combined rate (inflow+outflow)? The combined rate would be 10l outflow/hour. So time required will be 10/5 = 2 hours. Therefore, the outlet pipe should be opened at 9:30am + 6 hours  2 hours = 1:30pm. Any mistake here? Not sure how you got the numbers there. Combined rate is INFLOW of 1/51/15=2/15 pool/hour... We are interested on time needed for outlet pipe to compensate the excess of 1/5 of the pool.
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Re: A tank has both an inlet pipe and an outlet pipe. Working
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26 Sep 2014, 21:44
Inlet: 1/5 t/h Oulet: 1/15 t/h together working 1/51/15=2/15 t/h
2/15*6=12/15 filled, so 3/15 unfilled
3/15:1/15=3 hours of outlet should subtract. 63=3 hours
Answer: D



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Re: A tank has both an inlet pipe and an outlet pipe. Working
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30 May 2016, 12:14
Hi chetan2u, I am able to get the solution from the following method 1/5 * t + 2/15 * (6t) = 1 (Since the tank should be full after 6 hours) t/5 + 12/15 2t/15 = 1 12/15 + t/15 = 1 t/15 = 3/15 t=3 However, I want to discuss 1 more approach Inlet pipe rate= 1/5 outlet pipe rate= 1/15 Combined rate of Inlet and Outlet = 1/5  1/15 = 2/15 As, mentioned in the question the tank gets filled in 6 hours. We can say that Inlet pipe works for 6 hours and and Inlet = outlet pipe works for x hours So, I tried to make the equation as (1/5)*6 + (2/15)*x= What do I need to put on RHS Can you please assist. Thanks and Regards, Prakhar
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A tank has both an inlet pipe and an outlet pipe. Working
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17 Sep 2016, 23:01
PrakharGMAT wrote: Hi chetan2u, I am able to get the solution from the following method 1/5 * t + 2/15 * (6t) = 1 (Since the tank should be full after 6 hours) t/5 + 12/15 2t/15 = 1 12/15 + t/15 = 1 t/15 = 3/15 t=3 However, I want to discuss 1 more approach Inlet pipe rate= 1/5 outlet pipe rate= 1/15 Combined rate of Inlet and Outlet = 1/5  1/15 = 2/15 As, mentioned in the question the tank gets filled in 6 hours. We can say that Inlet pipe works for 6 hours and and Inlet = outlet pipe works for x hours So, I tried to make the equation as (1/5)*6 + (2/15)*x= What do I need to put on RHS Can you please assist. Thanks and Regards, Prakhar PrakharGMATwhen you say 2/15 it means the combined work of both inlet and outlet. then you cannot consider 1/5 work of inlet for 6 hours since it will overlap with the combined time. then equation will \(\frac{2}{15}x+\frac{1}{5}(6x)=1\)(Since the tank should be full after 68 hours) solving this equation will result \(x=3\).



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Re: A tank has both an inlet pipe and an outlet pipe. Working
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06 Oct 2018, 08:19
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Re: A tank has both an inlet pipe and an outlet pipe. Working
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