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Arithmetic Ability
Pipes and Cistern

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Q.

There is an empty reservoir whose capacity is 30 litres. There is an inlet pipe which fills at 5 L/min and there is an outlet pipe which empties at 4 L/min. Both the pipes function alternately for 1 minute. Assuming that the inlet pipe is the first one to function, how much time will it take for the reservoir to be filled up to its capacity?

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Correct choice: C

Explanation:

The work to be done = Capacity of reservoir  = 30 litres.  1st Minute ---> inlet pipe opened ---> 5 lit filled  2nd minute ---> inlet pipe closed; outlet pipe opened ---> 4 lit emptied In 2 minutes (5 litres - 4 litres = 1lit) is filled into the reservoir. It takes 2 minutes to fill 1lit ---> it takes 50 minutes to fill 25 litres into the reservoir.   In the 51st minute inlet pipe is opened and the reservoir is filled.
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