Fangfang sends Grandpa to the bus station, which runs every 40 minutes. 13 sent a car, Fangfang.

Suppose there is a bus every x minutes, the speed of the bus is a, and the speed of people is b,

The distance between every two adjacent buses is S=AX=8(A+B=40/3(A-B)).

From 8(A+B=40/3(A-B), we know that A=4B. If we substitute AX=8(A+B), we know that X= 10.

So the bus runs every 10 minutes.

(The distance between every two adjacent cars is fixed, the speed of the oncoming car relative to Siu Wah Yuk is A+B, and the speed of the coming car relative to Siu Wah Yuk is A-B). The explanation of the equation is in brackets.