A 45g steel marble collides obliquely with an identical stationary marble, and continues at 55° to its original direction. The collision is perfectly elastic. What is the angle between the direction taken by the target ball and the original direction of the incident ball?

Let's say that you are watching this from above, and the initial marble is going upwards.

The momentum along the horizontal remains zero.
The momentum along the vertical is the same as initially as per the pronciple of conservation of momentum.

Can you work this out? :)