# QA – Placement Quizzes | Pipes and Cisterns | Question 6

Three pipes A, B and C were opened to fill a cistern. Working alone, A, B and C require 12, 15 and 20 minutes respectively. Another pipe D, which is a waste pipe, can empty the filled tank in 30 minutes working alone. What is the total time (in minutes) taken to fill the cistern if all the pipes are simultaneously opened ?**(A)** 5**(B)** 6**(C)** 7**(D)** 8**Answer:** **(B)****Explanation:** Let the capacity of the cistern be LCM(12, 15, 20, 30) = 60 units.

=> Efficiency of pipe A = 60 / 12 = 5 units / minute

=> Efficiency of pipe B = 60 / 15 = 4 units / minute

=> Efficiency of pipe C = 60 / 20 = 3 units / minute

=> Efficiency of pipe D = 60 / 30 = 2 units / minute

=> Combined efficiency of pipe A, pipe B, pipe C and pipe D = 10 units / minute

Therefore, time required to fill the cistern if all the pipes are opened simultaneously = 60 / 10 = 6 minutes

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