Q. A cistern can be filled with water by a pipe in 5 hours and it can be emptied by a second pipe in 4 hours. If both the pipes are opned when the cistern is full, the time in which it will be emptied is :
Answer: 20 hours
Notes: According to the question, Cistern filled in 1 hour = 1/5 part Cistern emptied in 1 hour = 1/4 part When both pipes are opened simultaneously; Cistern emptied in 1 hour = $Latex \frac{1}{4}-\frac{1}{5}=\frac{5-4}{20}=\frac{1}{20}&s=1$ ∴The time in which it will be emptied = 20 hours. Therefore option [D] is correct.

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