Each morning during rush hour, 20,000 people need to travel from Baltimore to Wash- ington, D.C. There are only two modes of transportation available: rail (train) or road (car). If a person takes the train, the trip lasts 40 minutes per person. If x thousand people use the road, it takes 20 + 5x minutes per person to make the trip. (We're assuming all people travel separately; no carpools) (a) It is assumed that when people are left to their own devices, they will divide up between rail and roads so that the average travel time by road equals the average travel time by rail. How many people travel by car under this "equilibrium"? (b) Use calculus to find a solution that minimizes the average travel time for any person