Newton's laws of motion — practice question

Particles $A$ and $B$, with masses $0.8\,\text{kg}$ and $0.2\,\text{kg}$ respectively, are joined by a light inextensible string. $A$ rests on a horizontal plane. The string runs over a small smooth pulley $P$ at the edge of the plane, and $B$ is suspended freely. The horizontal part of the string, $AP$, has length $2.5\,\text{m}$. The particles are let go from rest, with both parts of the string taut.