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# Pipe A and B running together can fill a cistern in 6 minutes. If Pipe

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Intern
Joined: 07 Feb 2016
Posts: 6
Pipe A and B running together can fill a cistern in 6 minutes. If Pipe  [#permalink]

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Updated on: 12 Mar 2016, 07:35
1
4
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Difficulty:

45% (medium)

Question Stats:

73% (02:33) correct 27% (02:24) wrong based on 167 sessions

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Pipe A and B running together can fill a cistern in 6 minutes. If Pipe B takes 5 more minutes than Pipe A to fill the cistern, then time in which A and B can fill the cistern separately will be respectively?

A. 15 minutes, 20 Minutes.
B. 15 minutes, 10 Minutes.
C. 12 minutes, 7 minutes.
D. 25 minutes, 20 minutes.
E. 10 minutes, 15 minutes.

Originally posted by Why2settleForLess on 12 Mar 2016, 07:15.
Last edited by Bunuel on 12 Mar 2016, 07:35, edited 1 time in total.
Renamed the topic and edited the question.
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Pipe A and B running together can fill a cistern in 6 minutes. If Pipe  [#permalink]

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12 Mar 2016, 07:44
Why2settleForLess wrote:
Pipe A and B running together can fill a cistern in 6 minutes. If Pipe B takes 5 more minutes than Pipe A to fill the cistern, then time in which A and B can fill the cistern separately will be respectively?

A. 15 minutes, 20 Minutes.
B. 15 minutes, 10 Minutes.
C. 12 minutes, 7 minutes.
D. 25 minutes, 20 minutes.
E. 10 minutes, 15 minutes.

Let A and B be the times taken by pipes A and B respectively to fill the cistern on their own ---> B=A+5

Thus, based on the information provided,

Cistern filled in 1 minute by A alone + Cistern filled in 1 minute by B alone = Cistern filled in 1 minute

$$\frac{1}{A} + \frac{1}{B} = \frac{1}{6}$$

$$\frac{1}{A} + \frac{1}{A+5} = \frac{1}{6}$$

$$A^2-7A-30=0$$ --->$$A=10$$ minutes and $$B=A+5=10+5=15$$ minutes.

E is thus the correct answer.

FYI, options, B,C and D are automatically out as these options go against the given information of B=A+5, leaving options A and E in the mix.

Hope this helps.
Math Expert
Joined: 02 Aug 2009
Posts: 7200
Re: Pipe A and B running together can fill a cistern in 6 minutes. If Pipe  [#permalink]

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12 Mar 2016, 07:54
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1
Why2settleForLess wrote:
Pipe A and B running together can fill a cistern in 6 minutes. If Pipe B takes 5 more minutes than Pipe A to fill the cistern, then time in which A and B can fill the cistern separately will be respectively?

A. 15 minutes, 20 Minutes.
B. 15 minutes, 10 Minutes.
C. 12 minutes, 7 minutes.
D. 25 minutes, 20 minutes.
E. 10 minutes, 15 minutes.

Hi,
A short and sweet method is being provided by the choices avail..

Its clearly given that B takes 5 more min than A..
so the answer should be x, x+5..
B, C and D can be ELIMINATED as it gives A 5 more minutes..

choices LEFT
A. 15 and 20
E. 10 and 15.

If A is the faster and we have B too working at the same speed as A, time taken will be 15/2= 7.5 minutes, which is more than 6 minutes..
so In no way the answer can be 6 as combined effort
eliminate A

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Re: Pipe A and B running together can fill a cistern in 6 minutes. If Pipe  [#permalink]

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12 Mar 2016, 13:13
my gosh,
I got into all sorts of equations here..
It sure took more than 3 mins,
Manager
Joined: 09 Jun 2015
Posts: 92
Re: Pipe A and B running together can fill a cistern in 6 minutes. If Pipe  [#permalink]

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13 Mar 2016, 06:08
Why2settleForLess wrote:
Pipe A and B running together can fill a cistern in 6 minutes. If Pipe B takes 5 more minutes than Pipe A to fill the cistern, then time in which A and B can fill the cistern separately will be respectively?

A. 15 minutes, 20 Minutes.
B. 15 minutes, 10 Minutes.
C. 12 minutes, 7 minutes.
D. 25 minutes, 20 minutes.
E. 10 minutes, 15 minutes.

Supposing pipe A takes 'x' minutes and pipe 'B' takes y minutes, then A and B together will take x*y/(x+y) minutes.
By supposition, x*y = 6(x+y)
Plugging in values, we get option E
(Note option B is wrong because the values are given in the revers order)
CEO
Joined: 20 Mar 2014
Posts: 2639
Concentration: Finance, Strategy
Schools: Kellogg '18 (M)
GMAT 1: 750 Q49 V44
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Re: Pipe A and B running together can fill a cistern in 6 minutes. If Pipe  [#permalink]

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13 Mar 2016, 06:10
Mathivanan Palraj wrote:
Why2settleForLess wrote:
Pipe A and B running together can fill a cistern in 6 minutes. If Pipe B takes 5 more minutes than Pipe A to fill the cistern, then time in which A and B can fill the cistern separately will be respectively?

A. 15 minutes, 20 Minutes.
B. 15 minutes, 10 Minutes.
C. 12 minutes, 7 minutes.
D. 25 minutes, 20 minutes.
E. 10 minutes, 15 minutes.

Supposing pipe A takes 'x' minutes and pipe 'B' takes y minutes, then A and B together will take x*y/(x+y) minutes.
By supposition, x*y = 6(x+y)
Plugging in values, we get option E
(Note option B is wrong because the values are given in the revers order)

Reason used to eliminate B is the same one used to eliminate C and D as well.
Intern
Joined: 15 Jun 2013
Posts: 47
Schools: Ivey '19 (I)
GMAT 1: 690 Q49 V35
GPA: 3.82
Re: Pipe A and B running together can fill a cistern in 6 minutes. If Pipe  [#permalink]

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12 Apr 2017, 23:06
1
Let's do it without equations.
It is clear how B,C and D were eliminated.
Now we know that both pipes toghether fill the cistern in 6 minutes, so they fill 10 cisterns in an hour.
A) 15 and 20 minutes mean pipe A can fill 4 cisterns and B 3 cisterns in an hour. 7 cisterns toghether<10
E) 10 and 15 minutes mean 6 and 4 cisterns per hour or 10 toghether. E indeed.
Intern
Joined: 16 Jan 2018
Posts: 1
Re: Pipe A and B running together can fill a cistern in 6 minutes. If Pipe  [#permalink]

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16 Jan 2018, 09:26
1
Pipe A can fill an empty water tank in 3 hours while pipe B can fill the same tank in 6 hours. When the tank is full it can be emptied by pipe C in 8 hours. Pipe A and B are opened at the same time when the tank is empty. If one hour late, pipe C is opened find the total time taken to fill the tank.

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Re: Pipe A and B running together can fill a cistern in 6 minutes. If Pipe &nbs [#permalink] 16 Jan 2018, 09:26
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