The cost per hour for fuel to run a train is dollars, where v is the speed of the train in miles per hour. (Note that the 8 cost goes up as the square of the speed.) Other costs, including labor, are $480 per hour. How fast should the train travel on a 420-mile trip to minimize the total cost for the trip? Write the total cost C as a function of the speed v. C(v) = (Use integers or fractions for any numbers in the expression.)