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      Question

      ‘A’ alone can complete a work in 40 hours. ‘A’

      started the work alone and left after working on it for 20 hours. If ‘B’ completed the rest of the work in 20 hours then find the time taken by ‘A’ and ‘B’ together to complete the whole work.
      A 10 hours Correct Answer Incorrect Answer
      B 15 hours Correct Answer Incorrect Answer
      C 20 hours Correct Answer Incorrect Answer
      D 18 hours Correct Answer Incorrect Answer
      E None of these Correct Answer Incorrect Answer

      Solution

      Let the total work = 300 units Then, efficiency of ‘A’ = (300/40) = 7.5 units/hour Work completed by ‘A’ alone in 20 hours = 7.5 × 20 = 150 units Remaining work = 300 – 150 = 150 units So, efficiency of ‘B’ = 150/20 = 7.5 units/hour Therefore, combined efficiency of ‘A’ and ‘B’ = 7.5 + 7.5 = 15 units/hour So, time taken by ‘A’ and ‘B’ together to complete the work = (300/15) = 20 hours

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