The long-distance bus from A to B would have taken seven hours. After the opening of the expressway, the distance is nearly 30 kilometers, the average speed is increased by 30 kilometers, and it only takes 4 hours to arrive. Find out the distance between a and b on the expressway.

Solution: If the distance of expressway is S and the speed of expressway is V, then S=4V, S+30=7(V-30), and the solution is S=320, V=80, then the distance of expressway between A and B is 320km.

Settings; The speed is X7X=(X+30).4+30 X=50.