Two balls, 10 kg and 20 kg, approach each other with velocities of 20 m/s and 10 m/s respectively. What are their velocities after collision if the collision is perfectly elastic?
a) 20 m/s and 10 m/s
b) 10 m/s and 20 m/s
c) 15 m/s and 15 m/s
d) 5 m/s and 25 m/s