I can't distinguish which day is which in the graph, but the cumulative distribution functions tells you how many cars were below a certain speed. For example if the value of the graph at 50mph is 7 then it means 7 cars were going 50mph or slower. Since the maximum value of the graph is 50, you know there were 50 cars in total.
To determine how many cars were above a certain speed, find the number of cars at that speed and subtract this from 50. To find the proportion, divide the number of cars above the speed limit by the total number of cars. Hope it helps!