Concept: The question tests on the area of LCM/HCF
Solution:
The red light flashes every 60/3 = 20 seconds and the green light flashes every 120 / 5 = 24 seconds
LCM of 20 and 24 = 120 second = 2 minutes
So after the first flash, they will flash together 60/2 = 30 times