The lighter ball has a speed of $3~v_0$
Work Step by Step
Since the initial momentum of the system of two balls is zero, the magnitude of the lighter ball's final momentum $p_L$ should be equal to the magnitude of the heavier ball's final momentum $p_H$. We can equate the magnitudes of the momenta to find the speed $v$ of the lighter ball. $p_L = p_H$ $m~v = 3m~v_0$ $v = 3~v_0$ The lighter ball has a speed of $3~v_0$.