The angle of elevation of a jet plane from a point $\displaystyle A$ on the ground is $\displaystyle 60^{\circ}$. After a flight of $\displaystyle 30$ seconds, the angle of elevation changes to $\displaystyle 30^{\circ}$. If the jet plane is flying at a constant height of $\displaystyle 3600 \sqrt{3} \mathrm{~m}$, find the speed of the jet plane.

In \(\displaystyle \triangle A P B\) \[\begin{aligned} & \tan 60^{\circ}=\sqrt{3}=\frac{3600 \sqrt{3}}{x} \\ & \mathrm{x}=3600 \mathrm{~m} \end{aligned} \] In \(\displaystyle \triangle A Q C\) \(\displaystyle \tan 30^{\circ}=\frac{1}{\sqrt{3}}=\frac{3600 \sqrt{3}}{x+y}\) \(\displaystyle \mathrm{y}=7200 \mathrm{~m}\) speed of jet plane \(\displaystyle =\frac{7200}{30}=240 \mathrm{~m} / \mathrm{sec}\)