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Bunuel
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Rebaz

Bunuel
Official Solution:

­On Monday, a pipe working continuously filled an empty tank at an average (arithmetic mean) rate of 75 liters per hour. The next day, the pipe began filling another empty tank of the same volume. However, during this process, it was turned off for some time due to maintenance, resulting in a longer filling time. This reduced its average filling rate to 60 liters per hour. How many minutes per hour was the pipe turned off on Tuesday?

A. 10
B. 12
C. 15
D. 20
E. 25


There are many ways to solve this question; however, arguably the least error-prone method is to assume a specific volume for the tank. Let's say its volume is 300 liters (the least common multiple of 75 and 60). This would mean that without the maintenance interruption, the pipe would take \(\frac{300}{75} = 4\) hours to fill the tank. With the maintenance interruption, it would take \(\frac{300}{60} = 5\) hours. Thus, the pipe was turned off for 1 hour out of 5, which makes 1/5 of an hour per hour, or 12 minutes per hour.


Answer: B

Bunuel

Does On Monday, a pipe working continuously mean that the pipe worked during the whole 24 hours throughout Monday? If yes, then it seems we easily can get the volume of the tank by multiplying 75 liters per hour by 24 hours = 1800 liter is the tank capacity?

Is my approach regarding the tank capacity correct?


Thanks in advance!

This does not mean that the pipe worked for all 24 hours on Monday. It simply means that during the time the pipe was in operation on Monday, it worked continuously without any interruptions.
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Why are we saying 1 hour out of 5? I did get the solution till 4hrs and 5hrs with maintenance by taking a random volume for both the cases. However how do we understand that for how many hours the pipe was closed?
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Why are we saying 1 hour out of 5? I did get the solution till 4hrs and 5hrs with maintenance by taking a random volume for both the cases. However how do we understand that for how many hours the pipe was closed?

Hope that another discussion on this question will help clarify this issue: https://gmatclub.com/forum/gmat-club-ol ... 32248.html
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Hi Rebaz,

It doesn't matter. You can take it as 24 hours as well. Answer won't change. This is a ratio and proportion problem. What proportion of an hour was tap idle?

75min for 1 hour --> 5 parts
60min for 1 hour --> 4 parts

remaining 1 part is idle time which is 1/5*60min = 12min


Rebaz
Bunuel

Does On Monday, a pipe working continuously mean that the pipe worked during the whole 24 hours throughout Monday? If yes, then it seems we easily can get the volume of the tank by multiplying 75 liters per hour by 24 hours = 1800 liter is the tank capacity?

Is my approach regarding the tank capacity correct?


Thanks in advance!
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hi,
Bunuel
what went wrong in my approach?

Since volume is same in both days, we can have below equation-
75*t=60*(t+x)
which on solving gives x=t/4
i.e. 60*1/4=15 mins/hr.
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hi,
Bunuel
what went wrong in my approach?

Since volume is same in both days, we can have below equation-
75*t=60*(t+x)
which on solving gives x=t/4
i.e. 60*1/4=15 mins/hr.

If solving it that way, you should take the ratio of x to t + x, which represents the total time. This gives x/(t + x) = 1/5.
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I think this is a high-quality question.
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