The speed is x÷ 10/0.

(2) the distance is 300+x.

The speed is (x+300)÷20.

(3) The topic says: "The train runs at a constant speed", and the speed is of course the same!

(4) Solve the equation x ÷ 10 = (x+300) ÷ 20.

x=300

This train is 300 meters long.