Q. A cistern can be filled by pipe P in 3 hours and pipe Q in 5 hours and empty by pipe R in 2 hours. If all these pipes are open, then in how many hours the cistern will be full?
Answer: 30 hours
Notes: Because as per the question part of the cistern filled by all pipes in an hour $latex = \frac{1}{3}+\frac{1}{5}-\frac{1}{2}=\frac{10+6-15}{30}=\frac{1}{30}&s=1$ So, the cistern will be filled in 30 hours if all pipes are open. Hence option [C] is correct answer.

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