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      Question

      A, B and C start running at the same time and at the

      same point in the same direction in a circular stadium. A completes a round in 720 seconds, B in 1080 seconds and C in 1800 seconds. After what time will they meet again at the starting point?
      A 10 minutes Correct Answer Incorrect Answer
      B 9 minutes Correct Answer Incorrect Answer
      C 34 minutes Correct Answer Incorrect Answer
      D 18 minutes Correct Answer Incorrect Answer
      E 20 minutes Correct Answer Incorrect Answer

      Solution

      Factors of 720 = 2 × 2 × 2 × 2 × 3 × 3 × 5 Factors of 1080 = 2 × 2 × 2 × 3 × 3 × 3 × 5 Factors of 1800 = 2 × 2 × 2 × 3 × 3 × 5 × 5 LCM = 23 x 33 × 5   = 1080 seconds Thus, required time = 1080 seconds = 18 minutes

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