# QA – Placement Quizzes | Pipes and Cisterns | Question 10

• Difficulty Level : Medium
• Last Updated : 28 Jun, 2021

Two pipes A and B are connected to drain out a water tank. A alone can drain out the tank in 20 hours and B can drain 20 liters per hour. Find the capacity of the water tank given that working together, they require 12 hours to completely drain out the tank.
(A) 600
(B) 400
(C) 800
(D) 700

Explanation: Let the capacity of the tank be LCM (20, 12) = 60 units
=> Efficiency of A working alone = 60 / 20 = 3 units / hour
=> Efficiency of A and B working together = 60 / 12 = 5 units / hour
Therefore, Efficiency of B working alone = Efficiency of A and B working together – Efficiency of A working alone
=> Efficiency of B working alone = 5 – 3 = 2 units / hour
=> Time required by B alone to drain the tank = 60 / 2 = 30 hours

But we are given that B can drain the tank at the rate of 20 liters per hour.
Therefore, capacity of the water tank = 20 x 30 = 600 liters

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