Determine the number of automobiles with PremAir catalyst treated radiators needed to reduce the ozone concentration in an urban area by 10%. Assume the urban area covers 500 square miles, that there is little wind to add or remove ozone, that there is an inversion height of 2000 ft, that the air is well mixed so there is no spatial variation in concentrations. The initial concentration of ozone is 90 ppbv. Each car can treat 750,000 ft3/hr of air removing 70% of the ozone. If there are 250,000 cars operating at any given time, what percentage need to have the coating?