Three bulbs red, green and yellow flash at intervals 80 seconds, 90 seconds and 110
seconds. All three flash together at 8:00 am. When will these three bulbs flash together
again?

Dear Student,
The three bulbs flash at 80s, 90s and 110s respectively. Therefore, they will flash together at LCM of (80,90,110), that is after 7920 seconds = 132 minutes = 2 hours 12 minutes.
Therefore, after every 2 hours 12 minutes, all three bulbs will flash together.
When they flash together at 8am, they will again flash at 8 am+ 2hours 12minutes = 10:12am Answer 
Regards
 

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