posted by Alex .
A 550-g ball traveling at 8.0 m/s undergoes a head-on elastic collision with a 250-g ball traveling toward it also at 8.0 m/s. What is the speed of the 250-g mass after the collision?
You just have to use both momentum and energy equations, and do the messy algebra. Two equations, two unknowns (the two ball velocities after collision).
Enjoy the messy algebra.