Q1 of 4 Page 138

A red light flashes 3 times per minute green light flashes 4 times per minute and a yellow light flashes 6 times per minute at regular intervals. If all lights start flashing at the same time how many times do they flash together in each 75 minutes?

Interval of flashing of red light = 20 s


(3 times per minute)


Interval of flashing of Green light = 15 s


(4 times per minute)


Interval of flashing of red light = 10 s


(6 times per minute)


Since we have to find their common flash time, we need to find LCM of their intervals


LCM of intervals (20, 15, 10) = 60


⇒ Lights will blink together after intervals of 60s each


Lights flashed together in 75 minutes


= 75


So, lights will flash together 75 times in 75 minutes.


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