At the same instant that a 2.0-kg ball is dropped from 50 m above Earth, a second ball, with a mass of 1.0 kg, is thrown straight upward from Earth's surface with an initial speed of 20 m/s. They move along nearby lines and pass without colliding. What is the velocity of the center-of-mass of the two-ball system 0.5 seconds after the balls are released?