GMAT Club Olympics 2024 (Day 2): On Monday, a pipe working

Question

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

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Official Answer

B

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

Bunuel · Accepted Answer

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

This question was provided by GMAT Club
for the GMAT Club Olympics Competition

Win over $30,000 in prizes such as Courses, Tests, Private Tutoring, and more

Show Answer

Official Answer

B

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

Nalapatricia · Answer

How many minutes per hour was the pipe turned off on Tuesday?

Rate Pipe No. 1 = 75 Lt/hr

Rate Pipe No. 2 = 60 Lt/hr

Analyzing at what time both filled same amount of water:

75 (1) = 75 
75 (2) = 150
75 (3) = 225
75 (4) = 300

60 (1) = 60
60 (2) = 120
60 (3) = 180
60 (4) = 240
60 (5) = 300

So, difference between both rates is 1 hour. So, divide 60 Minutes (difference) / total time= 
=60/5 = 12 minutes off per hour. 
 Answer: B. 12

whatsarc · Answer

Lets assume the volume of the tank as 1500 L.

So A worked for 20hrs at 75L/hr
and B worked for 25hrs at 60L/hr.

B worked an extra of 5 hours, thats 300mins.

300 mins spread over 25hrs is 12mins per hour

Ans B - 12mins per hour

JLambert · Answer

I went a slightly different route than the other options.

Since it said minutes per hour, I pretended it took 1 hour. The difference between 60L and 75L is 15L, which is 1/5 of the potential 75L. That means I lost 1/5 of my fill time and 1/5 of an hour is 12 minutes.

k10890 · Answer

I did it this way:

Pipe takes 75L/hour. Next day average = 60L/hour. If the pipe was running for x hours, then,

75*x = 60
x = 60/75 = 4/5

So, the pipe was not running for 1/5 hour = 12 minutes

juliansorre · Answer

It's given that:

75*t = V (1) , where t is the time (in hours, without interruptions) required to fill the empty tank, and V is the volume of the tank.

With interruptions, the new time can be written as:
t' = t + y (2), where y is the total time during which the pipe was turned off.

We are asked the valeu of y/(t+y)*60. Ex.: if the pipe was turned off for the 2 and the total time was 6, (2/6*60) gives the number of minutes per hour that the pipe was off. This represents an average; it does not mean that in every single hour the pipe was off for exactly that amount of time

We also know:

t'*60 = V (3)

Thus,
(t+y)*60 = 75t (4)

It gives: y= t/4

(t/4)/(t+1/4*t)*60 = (1/4)*(4/5)*60 = 12.

GMAT Club Olympics 2024 (Day 2): On Monday, a pipe working

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