Try this

Let 小紅go from A to B.
At A 小紅see 2 cars from both ways, and when arrive at B小紅see 2 cars from both ways.

The key is to understand 10 cars 發車from each side, and 發車時間間隔is the time for a car runs from A to B.

Suppose two observers a at A, and b at B. Both a and b will see cars from both side with時間間隔 as 發車時間間隔. Both of them see 10 cars.

(It will be clearer if we think from the view of a. 迎麵來的公共汽車11th輛is still at B, which 小紅 will see. And 後麵追過來的公共汽車10th輛 is at A, which 小紅 will not see. So 小紅到家時，迎麵來的公共汽車數了11輛，後麵追過來的公共汽車數了9輛.)

So it takes 10 發車時間間隔s for小紅go from A at B. or 發車時間間隔is 1/10 of time for 小紅from A to B.

When the 迎麵發車9th car is at A, and the 迎麵發車10th car is at B. So it takes the 9th car 1 發車時間間隔 to run from A to B. So it takes 1/10 of the time for 小紅.

v=10x4千米每時=40千米每時

Futher questions will be:
到家時，迎麵來的公共汽車數了21輛，後麵追過來的公共汽車數了17輛。

到家時，迎麵來的公共汽車數了31輛，後麵追過來的公共汽車數了25輛。

The answer for the 2 questions should be 40千米每時.