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    Question

    A and B together can complete a task in 40 days, while A

    alone takes 100 days to finish it. When A, B, and C work together, they can complete the task in 20 days. Approximately how many days will it take for A and C to complete the task working together?
    A 40.5 days Correct Answer Incorrect Answer
    B 28.5 days Correct Answer Incorrect Answer
    C 35.5 days Correct Answer Incorrect Answer
    D 50.5 days Correct Answer Incorrect Answer
    E None of these Correct Answer Incorrect Answer

    Solution

    Let the total work be 200 units

    Efficiency of (A + B) = 200/40 = 5 units/day

    Efficiency of ‘A’ = 200/100 = 2 units/day

    Efficiency of (A + B + C) = 200/20 = 10 units

    Therefore, efficiency of (A + C) = Efficiency of (A + B + C) – {efficiency of (A + B) – efficiency of ‘A’}

    Or, Efficiency of (A + C) = 10 – (5 – 2) = 7 units/day

    Required time taken = 200/7 = 28.57 days ~ 28.5 days

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