A traffic signal repeats a cycle of green for 25 seconds, amber for 5 seconds and red for 30 seconds. A car reaches the signal at a random moment. What is the probability that the signal is green when the car arrives?

One cycle lasts 25 + 5 + 30 = 60 seconds and green fills 25 of them, so the probability is 25/60 = 5/12. Treating the three colours as equally likely gives 1/3, which ignores that they last for very different times; 1/2 is the probability of red and 1/12 that of amber. This is a length-of-time version of geometric probability.