A cistern can be filled with water by a pipe in 5 hours and it can be emptied by a second pipe in 4 ...........

Quantitative Aptitude ( CCS) Pipes&cistern

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 opened when the cistern is full, the time in which it will be emptied is

9 hours

18 hours

20 hours

20 1/2 hours

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Explanation:

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 = $\frac{1}{4}-\frac{1}{5}=\frac{5-4}{20}=\frac{1}{20}$
∴The time in which it will be emptied = 20 hours