A400-kg stopped at an intersection is rear-ended by a 1600-kg truck moving at 10 m/s. if the car was in neutral and the brakes were off, so that the collision is approximately elastic, find final velocity of the car if the truck was moving 5 m/s after the collision.

The sum of the momenta before and after the collision will be the same.  Before the car's momentum was 0 and the truck's was P=mv, P=1600*10=16000kgm/s
Afterward the truck has momentum P=1600*5=8000kgm/s
So the car gets the rest of what we started with Pcar=(16000-8000)=8000kgm/s
So the car's velocity was Pcar=mv,  Pcar/m = v,   8000/400=20m/s