Particles $P$ and $Q$ are attached to opposite ends of a light inextensible string which passes over a fixed smooth pulley. The system is released from rest with the string taut, with its straight parts vertical, and with both particles at a height of $2{\text{ }}m$ above horizontal ground. $P$ moves vertically downwards and does not rebound when it hits the ground. At the instant that $P$ hits the ground, $Q$ is at the point $X$, from where it continues to move vertically upwards without reaching the pulley. Given that $P$ has mass $0.9{\text{ }}kg$ and that the tension in the string is $7.2{\text{ }}N$ while $P$ is moving, find the total distance travelled by $Q$ from the instant it first reaches $X$ until it returns to $X$.