Answers

2015-05-06T10:59:26+05:30
Red light flash in interval  of 20 second
green ----------//-------------of 15 second
yellow ------------//---------- of 10 second
interval 75*60 second
lcm of (20,15,10) =60
that means every 60 second they flash together once!
75*60/60=75
they flash together 75 times
5 4 5