Can someone explain why the answer is this?
The question is the following:
A large locomotive with a mass 4 times that of the smaller motionless railroad car collides and couples together. What is their combines speed after the collision?

I saw my classmates working out and she said its this but I don't understand WHY its like this and not any other way:

4m x 5 + m x o = 4mv + mv
20m + 0.5mv
20m = 5mv
v = 20m/5m
v = 4km/h