Energy, work and power — practice question

Particles $A$ and $B$ have masses $m\,\text{kg}$ and $0.1\,\text{kg}$ respectively, with $m > 0.1$. They are fastened to the two ends of a light inextensible string, which runs over a fixed smooth pulley so that the particles hang vertically beneath it (see diagram). Initially, each particle is $0.9\,\text{m}$ above level ground, and the system is released from rest; during the time that both particles are moving, the string tension is $1.5\,\text{N}$. Particle $B$ does not reach the pulley.