Newton's laws of motion — practice question

Particles $A$ and $B$, with masses $0.35\,\text{kg}$ and $0.15\,\text{kg}$ respectively, are fixed to the ends of a light inextensible string that runs over a fixed smooth pulley. The system is initially at rest, with $B$ resting on the horizontal floor, the string taut and its straight sections vertical. $A$ is $1.6\,\text{m}$ above the floor (see diagram). $B$ is then released and the system starts to move; $B$ does not reach the pulley.