DRDO Multi tasking staff Model paper 4

Given data-
Cistern is full at a given time
Cistern can be filled with water by first pipe in 5 hours.
Part of cistern filled by first pipe in one hour = 1/5
It can be emptied by a second pipe in 4 hours.
Part of cistern emptied by second pipe in one hour = 1/4
When both the pipes are open, in one hour, part of tank filled =

1

5

−

1

4

=−

1

20

∴ in one hour, 1/20 part of cistern gets emptied.
∴ Entire cistern will be emptied in 20 hours.