# QA – Placement Quizzes | Pipes and Cisterns | Question 14

Two pipes can fill a tank in 10 hours and 12 hours resp. while third pipe empties the full tank in 20 hours. If all the three pipes operate simultaneously, in how much time the tank will be filled?**(A)** 7 hr. 30 min.**(B)** 7 hr. 25 min**(C)** 7 hr. 40 min**(D)** 7 hr. 50 min**Answer:** **(A)****Explanation:** Pipe A can fill a tank in x hours and pipe B can fill a tank in y hours and pipe C can empty it in z hours. If all the pipes are operate simultaneously, 1 hours of work of each of the three pipes = 1/x +1/y – 1/z

1 minutes work of each of the three pipes = 1/10 + 1/12 – 1/20 = 15/2 hours

= 7 ½ hour = 7 hours 30 minutes.

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