A railway wagon of mass 15 t is moving along a level track at 10 km/h when it collides and couples together with a second wagon of mass 30 t moving in the same direction at 5 km/h. if the two wagons couple together after the impact, what would be their common velocity?

Answer 1

just conserve momentum before and after the collision. You then have

15*10 + 30*5 = (15+30)v

Answer 2

But don't you convert it into the base units?