Momentum — practice question

Particles $P$ and $Q$, whose masses are $0.2\text{ kg}$ and $0.1\text{ kg}$ respectively, can move freely along a straight line on a smooth horizontal plane. $P$ is launched towards $Q$ with speed $5\text{ m s}^{-1}$. At the same instant, $Q$ is launched away from $P$ with speed $2\text{ m s}^{-1}$. When $P$ collides with $Q$, the particles coalesce.