A red light flashes 3 times per minute and a green light flashes 5 times in two minutes at regular intervals. - General Test


A red light flashes 3 times per minute and a green light flashes 5 times in two minutes at regular intervals. If both the lights start flashing at the same time, how many times do they flash together in each hour?


  • 30

  • 24

  • 20

  • 60





In one minute or 60 seconds, red light flashes 3 times, i.e. it flashes after every 20 sec. `[60/3 = 20]`.

Similarly, green light flashes in every `(2 xx 60)/5` 24 sec.

L.C.M. of (20, 24) = 120 sec.

Hence both will flash together after every 120 sec.

In 1 hour or 60 × 60 = 3600 seconds, they will flash together `3600/120` = 30 times.

Concept: Number System (Entrance Exam)
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