A complete cycle of a traffic light takes 60 seconds. During each cycle the light is green for 25 second, yellow for 5 second and red for 30 second. At a randomly chosen time, the probability that the light will not be green is

A randomly chosen instant within the 60-second cycle is equally likely to fall anywhere in the cycle (uniform within one period). The light is not green whenever it is yellow or red, i.e., for 5 + 30 = 35 seconds out of the 60-second cycle. Using seconds as the time unit is standard [1], and traffic signals operate in repeating cycles with allocated green/yellow/red splits [2]. Thus the required probability = 35/60 = 7/12, which corresponds to option 4.

Sources

1. [1] Science-Class VII . NCERT(Revised ed 2025) > Chapter 8: Measurement of Time and Motion > In a Nutshell > p. 118
2. [2] https://itre.ncsu.edu/wp-content/uploads/sites/2/2017/04/Development-of-Real-Time-Performance-Measurements-for-Closed-Loop-Signal-Systems-Using-Existing-Loop-Detectors.pdf