A car with a mass of 20 tons, moving at a speed of 0.56 m / s, collides with a platform

A car with a mass of 20 tons, moving at a speed of 0.56 m / s, collides with a platform with a mass of 8 tons, determine their speed after coupling.

I use the law of conservation of momentum:
m1 * v1 + m2 * v2 = m1 * v3 + m2 * v4
Given:
m1 = 20 t.
m2 = 8 t.
v1 = 0.56 m / s
v2 = 0 m / s
V = v3 = v4 – the total speed of the car and platform after coupling
20 * 10 ^ 3 * 0.56 = 20 * 10 ^ 3 * V + 8 * 10 ^ 3 * V
20 * 0.56 = V * (20 + 8)
V = 11.2 / 28
V = 0.4 m / s – speed after coupling the car and platform