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 :

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 :
[A]18 hours
[B]9 hours
[C]20.5 hours
[D]20 hours

20 hours
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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