A boy playing on the roof of a 10 m high building throws a ball with a speed of 10 m/s at an
angle of $30^\circ with the horizontal. How far from the throwing point will the ball be at the height
of 10 m from the ground?
\left[ {g = 10m/{s^2},\sin 30^\circ = {1 \over 2},\cos 30^\circ = {{\sqrt 3 } \over 2}} \right]