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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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