Question

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 :​

Answer 1

Hi..

(+) sign shows filling efficiency

(-) sign shows emptying efficiency

If A and B work simultaneously. Then A will fill 4 units / hour and B will empty 5 units/hour

If overall 1 unit/hour will be emptied.

Then full tank will empty in

total capacityA′s eff.+B′s eff.=204−5=20−1

=20 hours