I hate these kind of questions because they make me feel really dumb.
The light makes 3 revolutions in 60 seconds = 1 rev in 20 seconds assuming constant speed of revs.
Favorable outcome = 5 secs
Total outcome = 20 secs
Probability = 5/20 = 1/4.
Makes no sense and I have no clue how to do this. I would guess A in the exam and move the hell on!!!
Isn't that the probability that you are IN the light and there for the probability that you are NOT is 1 - 1/4 or 3/4
Another way of looking at it:
You are standing on the circumference of a circle. The light will cover the entire circumference in 20 seconds therefore it will only cover 25% in 5 seconds. That leaves 75% of the circumference that you could be standing on and remain in the dark.