Each runners time would be an average of 18.1
72.4/4=18.1
√- 81 doesn't result in a rational number because you can't get a negative from squaring any number. Instead you get a number with an imaginary number.
13 - √- 81 = 13 - 9i
Four ratios:
light to dark: 4:12
dark to light: 12:4
black to white: 2:4
white to gray: 4:10
gray to white: 10:4
any of those work.