Forces and equilibrium — practice question

The upper ends of two smooth inclined planes $A$ and $B$ meet at right angles. Plane $A$ makes an angle $\alpha$ with the horizontal, and plane $B$ makes an angle $\beta$ with the horizontal, where $\sin \alpha = \frac{63}{65}$ and $\sin \beta = \frac{16}{65}$. A small smooth pulley is fixed at the top of the planes, and a light inextensible string passes over it. Two particles $P$ and $Q$, each of mass $0.65\text{ kg}$, are connected to the string, one at each end. Particle $Q$ is held stationary at a point on the same line of greatest slope of plane $B$ as the pulley. Particle $P$ is hanging freely below the pulley in contact with plane $A$ (see diagram). Particle $Q$ is released, and the particles begin moving while the string stays taut.