A boy playing on the roof of a 10 m high building throws a ball with a speed of $10\;{\rm{m}}{{\rm{s}}^{ - 1}}$ at an angle of ${30^0}$ with the horizontal. How far from the throwing point will the ball be at the height of 10 m from the ground? $(g = 10{\rm{\;m}}{{\rm{s}}^{ - 2}},\sin 30^\circ  = 1/2,{\rm{\;cos\;}}30^\circ  = \sqrt 3 /2)$