Ball 1, with a mass of 20kg is moving to the right at 20 m/s. At what velocity should ball 2, with a mass of 40kg, move so that they both come to a standstill upon collision?

This is a conservation of momentum question. Initial momentum of the system is the momentum of ball 1 plus the momentum of ball 2. The final momentum of the system should be 0 since the balls stand still after the collision.

mv + mv = 0

mv = - mv

(20) (20) = - (40) Vi

400 = - 40Vi

Vi = - 10

So ball 2 should travel at 10m/s to the left