A railroad car with a mass of 1.99 104 kg moving at 2.80 m/s joins with two railroad cars already joined together, each with the same mass as the single car and initially moving in the same direction at 1.52 m/s.
(a) What is the speed of the three joined cars after the collision
(b) What is the decrease in kinetic energy during the collision?
(a) Have you tried using the equation of conservation of momentum?
M*2.80 + 2M*1.53 = 3M*V
M cancels out. You don't need know what it is, since they all have the same mass.
Solve for V, the final speed.
(b) You can easily calculate the initial and final total kinetic energies. Subtract final from initial.
There is still time. Get busy.