Ball a (mass of 0.5 kg) going +2.0 m/s collides with a stationary ball b (mass of 0.4 kg).
what is the velocity of ball a after the collision if ball b moves at +1.0 m/s after the
collision?

The correct answer is 1.2 m/s

: mv+mv=mv+mv

(0.5kg)(2m/s)+(0.4kg)(0m/s)=(0.5kg)v+(0.4kg)(1m/s)

= 1kg*m/s=(0.5kg)v+0.4kg*m/s

=1kg*m/s-0.4kg*m/s=(0.5kg)v

=0.6kg*m/s=(0.5kg)v

to solve for v we divide both side by 0.5kg

v=1.2m/s