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CAT Questions | Time and work | Pipes and Cisterns

The question is from Pipes and Cisterns. It is about a cistern with an inlet and an outlet pipe. Based on some details, how can we find out the flow rate of the outlet pipe.

Question 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

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Explanatory Answer

Method of solving this CAT Question from Pipes Cisterns and Work Time: Classic Pipes - Cisterns Question!!

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 * 12 = 6 litres/min > 5 litres/min

The question is "Which of the following can be the outflow rate?"

Hence, the answer is "12 litres/minute".

Choice C is the correct answer.


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