A proton of mass mP and a positively charged nucleus of mass 9mP are widely separated and traveling towards each other at the same speed of 1500 m/s along the same line. Because they are both positively charged, they repel each other and undergo a soft head-on collision. As you know, the electrostatic repulsive force is a conservative force so the 'collision' is perfectly elastic when we consider the speeds a long time before and a long time after the collision (when they are essentially no longer interacting).
What is the final speed of the proton (in m/s) when they are again widely separated? (i.e. when they exert negligible forces on each other, and their speeds are no longer changing.)
(Note: you don't actually need the mass of the proton here, only the mass ratio '8', since mP will cancel out of both of your equations.)