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    Question

    Three pipes are connected to a tank. Pipe A can fill it

    in 30 minutes, pipe B can fill it in 60 minutes, while pipe C can empty the completely filled tank in 40 minutes. Pipe A is opened first, pipe B after 10 minutes, and pipe C after 20 minutes. Find the total time taken to fill the tank.
    A 20 min 30 sec Correct Answer Incorrect Answer
    B 26 min 40 sec Correct Answer Incorrect Answer
    C 40 min 20 sec Correct Answer Incorrect Answer
    D 45 min 15 sec Correct Answer Incorrect Answer
    E 17 min 25 sec Correct Answer Incorrect Answer

    Solution

    Let the capacity of the tank be 120 litres. {LCM (30, 60 and 40)}
    So, efficiency of inlet pipe A = 120 ÷ 30 = 4 litres/minute
    And, efficiency of inlet pipe B = 120 ÷ 60 = 2 litres/minute
    And, efficiency of outlet pipe C = 120 ÷ 40 = 3 litres/minute
    In first 10 minutes, pipe A will fill 10 × 4 = 40 litres
    And, pipes A and B together will fill 10 × (4 + 2) = 60 litres
    So, required time = 10 + 10 + {(120 − 40 − 60) ÷ (4 + 2 − 3)}
    = 20 + (20 ÷ 3) = 26 minutes 40 seconds

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