a car of mass 1000kg moving at 20ms^1 hits the back of the lorry of mass 5000kg which is parked by the road side.if the car stick to the lorry after the collision,what is the common velocity?

m1 v1 = ( m1 + M ) V
V = m1 v1 / ( m1 + M ).

I assume that you know that the answer is obtained by using the principle of the conservation of momentum, correct?