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 :
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 =
∴The time in which it will be emptied = 20 hours.
Therefore option [D] is correct.