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    Question

    A can do a piece of work in 21 days and B in 28 days. If

    they together work on it for 5 days, then the fraction of the work that is left is :  
    A 8/15 Correct Answer Incorrect Answer
    B 31/45 Correct Answer Incorrect Answer
    C 3/4 Correct Answer Incorrect Answer
    D 7/12 Correct Answer Incorrect Answer

    Solution

    A = 21 days B = 28 days LCM of (A and B) = LCM of 21 and 28 = 84 units Efficiency of A = 4 units/day and B = 3 units/day Total = 4 + 3 = 7 units/day 5 days work of A and B is 7  ×  5 = 35 units Remaining work = 84 - 35 = 49 units Rest work / total work = 49/84 Required Fraction = 49/84 = 7/12 

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