A stone projected at an angle of $60°$ from the ground level strikes at an angle of $30°$ on the roof of a building of height $\text{'}h\text{'}$. Then the speed of projection of the stone is:

Let initial and final speeds of stone be $u$ and $v$.

Considering the vertical velocities

$\therefore {\left(v\sin {30}^0\right)}^2={\left(u\sin {60}^0\right)}^2-2gh$                  ……(1)

and $v \cos 30°=u \cos 60°$      …….(2)

solving 1 and 2 we get

$\mathrm{u}=\sqrt{3\mathrm{gh}}$