Momentum — practice question

Two particles, $P$ and $Q$, with masses $2m\,\text{kg}$ and $m\,\text{kg}$ respectively, are initially at rest on the same vertical line. Their heights above horizontal ground are $1\,\text{m}$ for $P$ and $2\,\text{m}$ for $Q$. $P$ is thrown vertically upward at speed $2\,\text{m s}^{-1}$, and at that same moment $Q$ is let go from rest.