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    Question

    A alone can assemble 40 mobile phones in 10 hours, while

    B alone can assemble the same number in 15 hours. Working together, in how many hours can A and B assemble 120 mobile phones?
    A 20 hrs Correct Answer Incorrect Answer
    B 12 hrs Correct Answer Incorrect Answer
    C 15 hrs Correct Answer Incorrect Answer
    D 18 hrs Correct Answer Incorrect Answer
    E None of these Correct Answer Incorrect Answer

    Solution

    A’s rate = 40 / 10 = 4 phones/hour B’s rate = 40 / 15 = 8/3 phones/hour Combined rate = 4 + 8/3 = 20/3 phones/hour Time for 120 phones = 120 ÷ (20/3) = 120 × 3 / 20 = 18 hours

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