A ball of mass 625 g traveling 4.00 m/s to the right collides with another ball of

Question:

A ball of mass 625 g traveling 4.00 m/s to the right collides with another ball of equal mass that is initially at rest. The first ball is at rest after the collision.
(a) What is the velocity of the second ball after the collision?
(b) What is the velocity of the second ball if the mass of the two balls of equal mass changes?