Momentum — practice question

Particles $P$ and $Q$, with masses $0.1\,\text{kg}$ and $0.3\,\text{kg}$ respectively, are initially stationary on a smooth horizontal plane. $P$ is sent directly towards $Q$ at speed $4u\,\text{m s}^{-1}$. At the same moment, $Q$ is sent directly towards $P$ at speed $u\,\text{m s}^{-1}$. Once $P$ and $Q$ have collided, $P$ travels at speed $2\,\text{m s}^{-1}$ and $Q$ travels at speed $4\,\text{m s}^{-1}$.