Momentum — practice question

Particles $P$ with mass $m\,\text{kg}$ and $Q$ with mass $0.2\,\text{kg}$ are able to move freely on a smooth horizontal plane. $P$ is launched at a speed of $2\,\text{m s}^{-1}$ towards $Q$, which is at rest. After the collision, $P$ and $Q$ travel in opposite directions with speeds of $0.5\,\text{m s}^{-1}$ and $1\,\text{m s}^{-1}$ respectively.