A TV tower has a height of 150m. By how much the minimum height (in meters) of the tower be increased so as to double the radius of  its coverage range?

Coverage range of a TV tower is given by 
$r=\sqrt{2hR}$
where h is height of tower and R is radius of earth
Coverage range  $r\propto \sqrt{h}$
To make  r double, h  is to be made 4 times.
So, the height of the tower is to be made 4 times of initial height =  $4\times 150=600m$
The height to be increased from $150\text{\hspace{0.17em}m}$  is  $600-150=450\text{\hspace{0.17em}m}$
Therefore, the correct answer is  450.0