Discussion

Explanation:

The flashing time for red light is = 60/3 = 20 sec

The flashing time for green light = 120/5 = 24 sec

So, they will flash together in LCM (20, 24) = 120 sec, i.e. 2 minutes, which is 60/2 = 30 times in an hour

Hence, option (a).

Alternatively,

Red light flashes 3 times in 1 minute i.e. 3 × 60 = 180 times in 1 hour.

Also, green light flashes 5 times in 2 minutes i.e. 5 × 30 = 150 times in 1 hour.

The number of times they flashed together in 1 hour is the HCF of 180 and 150, which is 30.

Hence, option (a).

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