The rules of elastic collisions are of course derived from the appropriate equations but conveptually you remember your three cases of a what happens when a moving object collides head on with a stationary object.
1. If the impacting one has less mass it bounces back. (Light balls bouncing back from heavy walls)
2. If the impacting object has more mass it will slow down but continue to move forward while setting the stationary object in motion with a higher velocity (Moving baseball bat hitting a ball and the swing continuing after the impact)
3. If the objects have the same mass the impacting object completely stops and the stationary object begins to move at exactly the same speed the impacting one had (Billiard balls colliding dead on or two <PERSON>'s cradle balls)
If collisions are not head on but at an angle then momentum is affected according to these rules along the normal direction of impact but momentum in the direction tangent to impact is unchanged.
To actually figure out the precice velocities in your problems you have to set up the appropriate equations but there are some reasons to at least reasonably expact the impacting billiard ball to bounce backwards. 621
Ideas which are not mathematically exact but conceptually reasonable:
Idea 1. The two stationary balls can be imagined as a compound object with twice the mass of the first object where now the moving ball hits an object with twice it's own mass and thus according to rule 1 should bounce back.
Idea 2. Track with a sketch what would happen if the two stationary balls would be ever so slightly offset so that the impacting ball hit the top one first instead of both at the same time. In this case you would have collisions of type 3 (equal mass) with the first collision deflecting the impacting ball downwards, hitting the lower ball and then ending up being deflected to the lower left with a component backwards. This is completely different from the perfect symmetry situation in that a minimal offset yields significant asymmetry and it's an interesting discussion on how to try and breach the gap but nevertheless contains backwards motion. 621