Adding all the seconds together would equal 40, and multiplying it by 2.5 will bring you to 100. So, there will be a 42.5% chance the light will be green (17x2.5 = 42.5), 50% it will be red (20x2.5 = 50), and 7.4% chance it will be yellow (3x2.5 = 7.4)
