Multiple choice general knowledge math & puzzles

Suppose we have three traffic light changing after 36,42 and 72 sec respectively. If the light had been switched on at 9:00 A.M sharp, at what time will they change simultaneously.

  1. 9:08:04 a.m

  2. 9:08:06 a.m

  3. 9:08:24 a.m

  4. 9:08:30 a.m
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

The lights change simultaneously at intervals equal to the LCM of 36, 42, and 72 seconds. LCM(36,42,72) = 504 seconds = 8 minutes 24 seconds. Starting at 9:00 AM, they will change together again at 9:08:24 AM. To find LCM: 36=2^2×3^2, 42=2×3×7, 72=2^3×3^2, so LCM=2^3×3^2×7=504.

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