Traffic light cycle of 60 s: Green = 25 s, Yellow = 5 s, Red = 30 s. At a random time, what is the probability the light is not green?

Introduction / Context:
Over a uniform 60-second cycle, the fraction of time in each color equals its probability at a random instant. We are asked for “not green.”

Given Data / Assumptions:
Total cycle = 60 s; Green = 25 s; Yellow = 5 s; Red = 30 s; random time uniformly across the cycle.

Concept / Approach:
P(not green) = (Yellow + Red) / Total.

Step-by-Step Solution:

Verification / Alternative check:
P(green) = 25/60 = 5/12 ⇒ complement 1 − 5/12 = 7/12.

Why Other Options Are Wrong:
1/3 or 1/4 do not match the given durations; 5/12 is P(green), not “not green.”

Common Pitfalls:
Mixing up complement or misreading the seconds per color.

Final Answer:
7/12