A red light flashes 3 times per minute and a green light

[sreeharimenon]

A red light flashes 3 times per minute and a green light flashes 5 times in two minutes at regular intervals. If both lights start flashing at the same time, how many times do they flash together each hour ?

30
60
45
180
210

[madboy]

The red light flashes 3 times per min i.e once every 20 secs
The green light flashes 5 times in 2 mins at regular intervals i.e once every 24 secs
If t secs is the interval at which they flash together then t is the LCM of 20 and 24 i.e 120
Therefore, the two lights flash together at an interval of 120 secs or 2mins and thus, 30 times in an hour

[efet]

r - 3 times / 1 min
g - 5 times / 2 min

so in 2 minutes;
r flashes at : 20sec - 40sec - 60sec - 80sec - 100sec - 120sec
g flashes at : 24sec - 48sec - 72sec - 96sec - 120sec

in 2 minutes, they flash together only 1 time.
1 hour = 60 minutes / 2 minutes = 30.