What Time will They Ring Together Again?

Four electrical machines ring bells after 30 minutes, 60 minutes, 90 minutes and 120 minutes respectively. If all the bells rang together at 12 noon, then at what time will they ring together again?

  1. 6 p.m.
  2. Midnight 12
  3. 12 noon
  4. 6 a.m.
Anurag Mishra Professor Asked on 7th September 2015 in Maths.
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  • 1 Answer(s)

    Answer: (1) 6 p.m.

    Explanation:-
    Factors of 30 = 2 x 3 x 5
    Factors of 60 = 2 x 2 x 3 x 5
    Factors of 90 = 2 x 3 x 3 x 5
    Factors of 120 = 2 x 2 x 2 x 3 x 5

    Then, the L C M of 30, 60, 90 and 120 = 2 x 2 x 2 x 3 x 3 x 5
    = 360

    They ring together again after 360 min = 360/60 = 6 hr.

    They will be ring together again at 6 p.m.

    Hence, the answer is (1) 6 p.m.

    Anurag Mishra Professor Answered on 10th September 2015.
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