1999 AMC 8 Problems/Problem 10

problem

A complete cycle of a tra±c 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, what is the probability that the light will NOT be green? (A) 1/4 (B) 1/3 (C) 5/12 (D) 1/2 (E) 7/12

solution

(E) 7/12: time not green/total time = (R + Y)/(R + Y + G) = (35/60) = (7/12)

OR

The probability of green is (25/60) = (5/12) So the probability of not green is 1- (5/12) = (7/12) .