From the meaning of the title, the more cars for eight people, the more money you can save, so try to arrange cars for eight people first, 36/8 = more than four people, so rent a car for four people.

Total cost: 4*300+ 1*200= 1400 yuan.

Answers to supplementary questions:

The speed of a is x, then b is 2x.

(x+2x)*2.5=45

X = 6 km/h

Answer: A walks at a speed of 6 kilometers per hour.