At a certain instant, a ball is thrown downward with a velocity of 8.0 m/s from a height of 40 m. At the same instant, another ball is thrown upward from ground level directly in line with the first ball with a velocity of 12 m/s. Find
(a) the time when the balls collide and
(b) the height at which they collide. Take g = 10 m/s^2