Four railroad cars, each of mass 2.80 104 kg, are coupled together and coasting along horizontal tracks at sp

Question

Four railroad cars, each of mass 2.80 104 kg, are coupled together and coasting along horizontal tracks at speed vi toward the south. A very strong but foolish movie actor riding on the second car uncouples the front car and gives it a big push, increasing its speed to 4.15 m/s south. The remaining three cars continue moving south, now at 1.80 m/s.

Find the initial speed of the cars.
How much work did the actor do?

Answer 1

...What was the initial speed vi?
From conservation of momentum,
4*m*vi = m*4.15 + 3*m*1.8
vi = (m*4.15 + 3*m*1.8)/(4*m) = (4.15 + 3*1.8)/4
EDIT: So you also need the work done.
m = 2.8E4
KEi = 4*m*vi^2/2
KEf = m*4.15^2/2 + 3*m*1.8^2/2
Work done = KEf - KEi