UPSC Prelims 2002 · General Studies
Question 1: Basic Numeracy
A complete cycle of a traffic light takes 60 seconds. During each cycle the light is green for 25 seconds, yellow for 5 seconds and red for 30 seconds. At a randomly chosen time, the probability that the light will not be green is
AnswerThe answer is (d).
The light is green for 25 s of each 60 s cycle, so it is not green for 60 - 25 = 35 s (5 s yellow plus 30 s red). Probability = 35/60 = 7/12. For such questions, find the favourable time (or its complement) and divide by the whole cycle. Check: P(green) = 25/60 = 5/12, and 1 - 5/12 = 7/12.
Why each option is right or wrong
- (a) 1/3 would be 20 s of the cycle; the not-green time is 35 s.
- (b) 1/4 would be only 15 s of the cycle.
- (c) 5/12 is the chance that the light IS green (25/60), the opposite of what is asked.
- (d) 35/60 = 7/12: yellow plus red time over the whole cycle.
Explanation: written by UPSC Grade, not by UPSC.
Source: UPSC, Civil Services (Preliminary) Examination 2002, General Studies. UPSC's own paper isn't online; the words are from a published compilation. The paper on upsc.gov.in.