Question

3 bulbs red green n yellow flash at intervals 80 sec,90 sec. & 110 sec all flash together at 8 am. whn will they flash together again?
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Answer 1

the red light will flash from first time in one minute

60/3 = 20 sec.

that means it will flash at an interval each time 20 second 40 second 60 second 80 second 100 second and so one (multiple 20 )

simply the green light will flash for the first time in 2 minut

120 / 5 = 24 sec.

that mean it will flash at an interval of 24 each second time = 24 second 48 second

72 second 96 second 120 2nd 144 2nd and so on (multiple of 24 )

not taken the common multiple of both red and green light which is = 120

marke it easier by simply takeing in the LCM of 20 and 24 = 120

therefrom both the light will flash together from the first time in 120 sec.

as we know and our as 3600 seconds

therefore the number of of 120 seconds in one hour is = 3600/120= 30

that is the will flash together from 30 time in one hour

this is your answer

hope you understand