Question

In a series set, there are red lights and blue lights. All red lights glow in every 4 seconds and green lights glow in every 5 seconds. What is the least time in which both the red lights and green lights glow together?

Answer 1

Answer:

This is an application of LCM

Red light flashes 3 times per minute or once every 20 seconds.

So, the time taken for successive flashing is denoted by 20,40,60,80,100,120…. seconds

Green light flashes 5 times every two minutes or once every 24 seconds

So, the time taken for successive flashing is denoted by 24,48,72,96,120….seconds

The common term in both is 120. So, both Red & Green lights will flash together after every 120 seconds.

This is also denoted by LCM(20,24)

Therefore, they will flash together for :

60 x 60/120 = 30 times every hour.

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