1999 AMC 8 Problems/Problem 10
Okay, so the whole cycle, 
seconds, is our denominator. The light is green for 25 seconds in the cycle, so it is a color other than green for 35 seconds. So, the probability that it will not be green at a random time is 
. This can be simplified, by factoring out the factor of 
common to the numerator and the denominator, to 
.
