From a photograph one observs that on a level section of highway 20% of the vehicles are trucks, 80% are cars, and that there are 50 vehicles per mile of highway. The trucks travel at 40 mph, the cars at 50 mph. This highway also has a section with a steep grade on which the speed of the trucks drops to 20 mph, and the speed of the cars to 40mph. No vehicles enter or leave the observed sections of the highway (except at the ends), and the flows are stationary. There is no traffic congestion, gueuing or vehicle backup on this highway. Determine the percentage of trucks as seen by a stationary observer on the grade.