# CAT Practice : Pipes cisterns, Work time

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Find a complete "Null System". Work on the question after that.

## Fill pipes and drain pipes

Q.5: A fill pipe can fill a tank in 20 hours, a drain pipe can drain a tank in 30 hours. If a system of n pipes (fill pipes and drain pipes put together) can fill the tank in exactly 5 hours, which of the following are possible values of n (More than one option could be correct)?
(1)    32       (2)    54       (3)    29       (4)    40

1. 1 and 2 only
2. 1 and 3 only
3. 2 and 4 only
4. 2 and 3 only

Choice D. 2 and 3 only

## Detailed Solution

3 fill pipes cancel out 2 drain pipes. Plus, you need an additional 4 fill pipes fill the tank in 5 hours. so the answer has to be 5k + 4.
Both 54 and 29 are possible.
Choice (D)

Correct Answer: 2 and 3 only

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## More questions from Pipes Cisterns, Work Time

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.