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 ? [$g = 10\; m/s^2, \; \sin 30^{\circ} = 1/2,\; \cos 30^{\circ} = \sqrt{3}/2]$
