For two masses $m_{1}$ and $m_{2}$ with initial velocities $u_{1},u_{2}$ and final velocities $v_{1},v_{2}$, conservation of momentum gives—

• a$m_{1}u_{1}=m_{2}v_{2}$
• b$m_{1}u_{1}+m_{2}u_{2}=m_{1}v_{1}+m_{2}v_{2}$
• c$m_{1}+m_{2}=v_{1}+v_{2}$
• d$u_{1}-u_{2}=v_{1}-v_{2}$