Q: A traffic light has a cycle through green, yellow, red, green, yellow, red, etc.

Red and green are each 30 seconds with a 3 second yellow inbetween.

What is the probability that while Amir is watching during a random 3 second interval that the color changes?

A:

9/63 = 1/7

It is a 63 second cycle.

The light changes at t=30, t = 33 and t = 63 given three three second intervals:

[27,30] and [30, 33] and [60,63] is a total of 9 seconds out of the 63 second interval