Solve this :

Q. An aeroplane, when flying at a height of 4000 m from the ground passes vertically above another 
aeroplane at an instant when the angles of elevation of the two planes from the same point on the 
ground are 60^o and 45^o respectively. Find the vertical distance between the aeroplanes at that instant.
$\left(\mathrm{Take} \sqrt{3}=1.73\right)$