A partical A is projected with an initial velocity of $60 m/s$ at an angle $30 ^{\circ}$ to horizontal . At the same time a second particle B is projected in the opposite direction with initial speed of $50 m/s$ from a point at a distance of $100 m$ from A. If the particle collide in mid air. find the angle of projection $\alpha$ of particle B.$(g=10 m/s^2)$