# QA – Placement Quizzes | Pipes and Cisterns | Question 15

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:**(A)** 60 hr**(B)** 50 hr**(C)** 80 hr**(D)** 90 hr**Answer:** **(D)****Explanation:** 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.

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