A cistern can be filled in 9 hours but it takes 10 hours due to a leak in its bottom. If the cistern is full, then the time that the leak will take to empty it is:
Answer: D
If A can fill a tank in x hours and B can fill a tank in y hours. Then the tank will be filled (empty) in = (xy / y-x) hours
Time taken by A = 9 hours
Time taken by B = 10 hours
Time taken by A and B = (9×10 / 10-9) = 90 hours.