A balloon rises from rest with a constant acceleration $g/8$. A stone is released from it when it has risen to height h. The time taken by the stone to reach the ground is

The velocity of balloon at height h,$v=\sqrt{2\left(\frac{g}{8}\right)h}$

When the stone released from this balloon, it will go upward  with velocity v =$\frac{\sqrt{gh}}{2}$ (Same as that of balloon). In this condition time taken by stone to reach the ground

$t=\frac{v}{g}\left[1+\sqrt{1+\frac{2gh}{v^2}}\right]$$=\frac{\sqrt{gh}/2}{g}\left[1+\sqrt{1+\frac{2gh}{gh/4}}\right]$

$=\frac{2\sqrt{gh}}{g}=2\sqrt{\frac{h}{g}}$