Solution: Let the bus speed be Xkm/h, then the car speed is X+70÷2.5=X+28km/h, which is deduced from the meaning of the question:

2.5[X+(X+28)]=420，

5X+70=420

X=70 X+28=98 .

Answer: The average speed of cars is 98 kilometers per hour, and the average speed of buses is 70 kilometers per hour.