An aeroplane flying horizontally $1\;km$ above the ground is observed at an angle of elevation of $60^{\circ}$. After 10 seconds, its elevation is observed to be $30^{\circ}$. Find the speed of the aeroplane in km/hr

Question

$\begin{array}{1 1} 415.68\;km/hr \\ 510.68\;km/hr \\ 450\;km/hr \\ 425\;km/hr \end{array} $