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


  • Correct Answer
    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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  6. Pipes Cisterns - Algebra
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.