CAT Practice : Pipes cisterns, Work time

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Classic Pipes - Cisterns Question!!


    Q.16: A cistern of capacity 40 litres has an inlet and an outlet pipe. When both the pipes are opened at once, it takes 8 minutes to fill the cistern. However, if the outflow rate is increased 1.5 times, the cistern never gets filled. Which of the following can be the outflow rate?
    1. 8 litres/minute
    2. 6 litres/minute
    3. 12 litres/minute
    4. 9 litres/minute


  • Correct Answer
    Choice C. 12 litres/minute

Detailed Solution

Initially, net flow in the tank = 40 litres/ 8 minutes = 5 litres/minute

Now, the increase in outflow should be greater than 5 litres/min for the tank to fill up

Increase in outflow = 1.5x – x = 0.5x

Only in option c) 0.5 X 12 = 6 litres/min > 5 litres/min

Correct Answer: 12 litres/minute

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Mathematicians are creatures of habit. This topic has been called Pipes and Cisterns because someone named it like it long ago. Pipes and Tanks would be much better - at least makes it sound like wartime.