A 15 kg ball moving to the right at 2.0 m/s makes an elastic head-on collision with a 30 kg ball moving to the left at 1.5 m/s. After the collision, the smaller ball moves to the left at 3.0 m/s. Assume that neither ball rotates before or after the collision and that both balls are moving on a frictionless surface. What is the velocity of the 30 kg ball after the collision?