Momentum — practice question

Particles $P$, with mass $0.4\,\text{kg}$, and $Q$, with mass $0.5\,\text{kg}$, are free to move on a smooth horizontal plane. $P$ and $Q$ travel straight towards one another at speeds $2.5\,\text{m s}^{-1}$ and $1.5\,\text{m s}^{-1}$ respectively. After the collision between $P$ and $Q$, the speed of $Q$ is twice that of $P$.