2 blocks A and B of masses $m$ and $2m$ respectively are connected by a spring of spring constant K the masses are moving to the right with an uniform velocity $v_0$ each, the heavier mass leading the lighter one. The spring has its natural length during this motion block B collides head on with a 3rd Block C of mass $2m$ which was initially at rest.

- What is the velocity of B just after the collision ?
- What is the velocity of the center of mass?
- What is the maximum compression of the spring after the collision?

What so far I concluded is that I thought of using the reduced mass concept and reduced mass $\frac {2}{3}$m. $v_0$ =-$\frac {8}{3}mv$ where $v$ is the velocity of the center of mass.

But I am not so confident that I am right because I am unable to identify whether the external force is 0 or not. Moreover is it okay to use reduced mass concept? and how to think about the length of spring and its calculation?