Newton's laws of motion — practice question

Masses $A$ and $B$, with masses $m\,\text{kg}$ and $(1 - m)\,\text{kg}$ respectively, are joined to the two ends of a light inextensible string that runs over a fixed smooth pulley. The system is let go from rest, with the straight sections of the string initially vertical. $A$ moves vertically downwards, and $0.3\,\text{s}$ afterwards its speed is $0.6\,\text{m s}^{-1}$.