# QA – Placement Quizzes | Pipes and Cisterns | Question 1

Two outlet pipes A and B are connected to a full tank. Pipe A alone can empty the tank in 10 minutes and pipe B alone can empty the tank in 30 minutes. If both are opened together, how much time will it take to empty the tank completely?**(A)** 7 minutes**(B)** 7 minutes 30 seconds**(C)** 6 minutes**(D)** 6 minutes 3 seconds**Answer:** **(B)****Explanation:** Let the capacity of the tank be LCM(10, 30) = 30 units.

=> Efficiency of pipe A = 30 / 10 = 3 units / minute

=> Efficiency of pipe A = 30 / 30 = 1 units / minute

=> Combined efficiency of pipe A and pipe B = 4 units / minute

Therefore, time required to empty the tank if both pipes work = 30 / 4 = 7 minutes 30 seconds

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