Speed * Time 30= (vehicle length+bridge length 380)

Vehicle speed and length can be obtained by simultaneous equations.

As for why the distance is the length of the car+the length of the bridge, it can be understood that the car can not be regarded as a point (particle), and the car is very long. The crossing time begins when the front of the car enters the bridge and ends when the rear of the car leaves the bridge. It consists of two sections, one is from the beginning to the end of the bridge, the other is from the beginning to the end of the bridge, and the length is the length of the car. Is that understandable?