the green light flashes every 3 seconds,the red every 5 seconds and white at 10 seconds. iF they flash at 12 noon.When will they flash together again in a minute? in an hour?

Answer 1

Both, in a minute and in an hour.

The LCM of 3, 5 and 10 is the same as the LCM of 3 and 10 because 5 divides 10 (5 is a factor of 10).
The LCM of 3 and 10 is 30.
So the soonest the will flash together is 30 seconds.

Answer 2

The LCM of 3, 5 and 10 is the same as the LCM of 3 and 10 because 5 divides 10 (5 is a factor of 10).
The LCM of 3 and 10 is 30.
So the soonest the will flash together is 30 seconds.