All three ideas are equivalent- 

• Conservation of momentum
• Newtonís Second Law
• Center of mass motion

Let's look at a couple systems:

Consider two astronauts in space.

Here's the "answer".

Consider a truck ccolliding with a stationary car.

The initial momentum is 60,000 kg m/s², and the final momentum is the sum of the truck's (45,000 kg m/s²) and the car's (15,000 kgm/s²).
 

Or, a car colliding with a stationary truck.

Here the initial momentum is 20,000 kg m/s², and the final momentum is the sum of the truck's (15,000 kg m/s²) and the car's (5,000 kgm/s²).