The demand curve facing a perfectly competitive firm is a horizontal straight line equal to the equilibrium price of the entire market because a perfectly competitive firm can sell all units brought to market at the same price.
It takes eight hours for a 600 km journey, if 120 km is done by train and the rest by car. It takes 20 minutes more, if 200 km is done by train and the rest by car. The ratio of the speed of the train to that of the car is: