Forces and equilibrium — practice question

Three blocks $P$, $Q$ and $R$, with masses $25\,\text{kg}$, $20\,\text{kg}$ and $m\,\text{kg}$ respectively, are held in equilibrium by three light inextensible strings $OP$, $OQ$ and $OR$. The strings $OP$ and $OR$ each pass over a small fixed smooth pulley, $A$ and $B$ respectively, and $P$ and $R$ hang vertically beneath the pulleys. Block $Q$ hangs vertically below point $O$. The angle between $OA$ and the vertical is $30^\circ$ and the angle $BOQ$ is $\alpha^\circ$ (see diagram).