A pipe can fill a cistern in 9 hours. Due to a leak in its bottom, the cistern fills up in 10 hours. If the cistern is full, in how much time will it be emptied by the leak ?

• let the amount of work equivalent to filling a cistern = 90 units

As it is mentioned that A takes 9 hours to fill the tank then it means :

A's 1 hour work = 10 units

As mentioned that due to hole the whole tank fills up in 10 hours then the amount of work distorted by the hole every hour = 1 unit .

So when a full tank of 60units is filled ,time taken to empty that because of hole = $$\frac{90}{1}$$ = 90hours