An electric pump can fill a cistern in 6 hours. Because of a leak in the cistern it was taking 9 hrs to fill the cistern. Find the time in which the leak can drain all the water of the cistern when full.

An electric pump can fill a cistern in 6 hours.

Assuming the capacity of the cistern to be 90 litres.

Thus, the electric pump can fill the cistern 90 litres in 6 hours or 15 litres in one hour.

Because of a leak in the cistern, it was taking 9 hrs to fill the cistern.

Therefore, the total volume filled in 1 hour = 90/9 = 10 litres per hour.

We know the pump can fill 15 litres in one hour; therefore, because of the leak, 5 litres of water are removed every hour.

If the tank is filled, it contains 90 litres.

The time taken by the leak to empty = 90/5 = 18 hours.