Newton's laws of motion — practice question

Particles $A$ and $B$, whose masses are $2.4\,\text{kg}$ and $1.2\,\text{kg}$ respectively, are joined by a light inextensible string that passes over a fixed smooth pulley. $A$ is initially at a height of $2.1\,\text{m}$ above a horizontal plane and $B$ is $1.5\,\text{m}$ above the plane. The particles hang vertically and are released from rest. In the motion that follows, $A$ arrives at the plane and does not rebound, and $B$ does not reach the pulley.