Direction of Masses in Collision

[minimario]

Homework Statement

2. Homework Equations

Elastic collision: KE conserved

The Attempt at a Solution

I don't think there's really anything to work out...why would the 2M block move right as well?

 

[minimario]

Here's another problem on the same topic, that I can't decide

 

[HallsofIvy]

In both, use "conservation of momentum" and "conservation of energy" to get two equations to solve for the two velocities.