Momentum — practice question

Three particles $P$, $Q$ and $R$, with masses $0.1\,\text{kg}$, $0.2\,\text{kg}$ and $0.5\,\text{kg}$ respectively, are initially at rest in a straight line on a smooth horizontal plane. Particle $P$ is sent towards $Q$ with speed $5\,\text{m s}^{-1}$. Once $P$ and $Q$ have collided, $P$ rebounds at speed $1\,\text{m s}^{-1}$. $Q$ then collides with $R$. Immediately after the collision with $Q$, $R$ starts moving with speed $V\,\text{m s}^{-1}$.