2014 THREE(d)

A container is winched up vertically from a point P at a constant rate of $1.5\mathrm{m}{\mathrm{s}}^{-1}$.
It is being observed from point Q, which is $20\mathrm{m}$ horizontally from point P.
$\theta$ is the angle of elevation of the container from point Q.

At what rate is the angle of elevation increasing when the object is $20\mathrm{m}$ above point P?

*Show any derivatives that you need to find when solving this problem.*