A 0.200 kg croquet ball makes an elastic head-on collision with a second ball initially at rest. The second ball moves off with half the original speed of the first ball.

(a) What is the mass of the second ball?

(b) What fraction of the original kinetic energy (KE/KE) gets transferred to the second ball?

T0 solve this, write the conservation of momentum equations, and the conservation of energy. You have two equations, two unknowns.