A new tollway is built. For the first month there is no toll to use the road and each day 4800 cars use it. After the first month it is estimated that for each dollar increase in the toll, 800 fewer cars will use the tollway. At what price do you think the company should set the tolls in order to make the most money?
if there are x dollar increases, then the revenue (price * # cars) is
(4800-800x)(x) = 4800x - 800x^2
That's just a parabola, so the maximum revenue is at the vertex.