A car of mass 0.225 kg traveling with a speed of 0.50 m/s links up with another car of mass 0.450 kg...

Here is the complete question:
A model-train car of mass 0.225 kg traveling with a speed of 0.50 m/s links up with another car of mass 0.450 kg that is initially at rest
What is the speed of the cars immediately after they have linked together?
____m/s
Find the initial kinetic energy.
____mJ
Find the final kinetic energy.
____mJ

Using Conservation of momentum:
A) (.225*.5)=((.225+.45)V) solve for V
*add mass together since it is an inelastic collision
V=.167m/s

B) Using Energy Theorem
K_i=(1/2)mv^2
Initial kinetic energy= (1/2)(.225)(.5)^2
Initial kinetic energy= .028J

C) Using Energy Theorem:
K_f=(1/2)mv^2 m=(.225+.45) v=(.167)
Final kinetic energy = .009J

*Joules Still need to be converted to mJ
*Energy is lost due to the fact that the system is inelastic