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Runners or bells meet again: LCM of their intervals

This type of question, asked 3 times: CDS 1 2019, CDS 2 2021, CDS 1 2023. All 3 questions below, oldest first.

Try each one first, then open its answer.

  1. Question 1

    CDS 1 2019

    17
    XX, YY and ZZ start at same point and same time in the same direction to run around a circular stadium. XX completes a round in 252 seconds, YY in 308 seconds and ZZ in 198 seconds. After what time will they meet again at the starting point ?

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  2. Question 2

    CDS 2 2021

    15
    Three runners are running in a circular track, and they complete one round in 20, 30 and 35 minutes respectively. When will they next meet at the starting point ?

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  3. Question 3

    CDS 1 2023

    30
    There are four bells which ring at an interval of 15 minutes, 25 minutes, 35 minutes and 45 minutes respectively. If all of them ring at 9 A.M., how many more times will they ring together in the next 72 hours?

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