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    Question

    β€˜A’ alone can complete a work in 50 hours. β€˜A’

    started the work alone and left after working on it for 30 hours. If β€˜B’ completed the rest of the work in 20 hours then find the time taken by β€˜A’ and β€˜B’ together to complete the whole work.
    A 25 hours Correct Answer Incorrect Answer
    B 20 hours Correct Answer Incorrect Answer
    C 22 hours Correct Answer Incorrect Answer
    D 24 hours Correct Answer Incorrect Answer
    E None of these Correct Answer Incorrect Answer

    Solution

    Let the total work = 280 units Then, efficiency of β€˜A’ = (280/50) = 5.6 units/hour Work completed by β€˜A’ alone in 30 hours = 5.6 Γ— 30 = 168 units Remaining work = 280 – 168 = 112 units So, efficiency of β€˜B’ = 112/20 = 5.6 units/hour Therefore, combined efficiency of β€˜A’ and β€˜B’ = 5.6 + 5.6 = 11.2 units/hour So, time taken by β€˜A’ and β€˜B’ together to complete the work = (280/11.2) = 25 hours

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