Let two balls collide at a height s from the ground after t second when second ball is thrown upwards. ∴ Time taken by first ball to reach the point of collision = (t + 2) s s = 39.2 (t + 2) +
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(- 9.8) (t+2)2 = 39.2 (t + 2)-4.9 (t+2)2 ...(i) For second ball s = 39.2t +
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(−9.8)t2 = 39.2t - 4.9 t2 ... (ii) From eqs. (i) and (ii) 39.2 (t + 2) - 4.9 (t+2)2 = (39.2) t - 4.9 t2 On solving we get, t = 3s FromEq. (ii), s = 39.2x 3-4.9 x (3)2 = 117.6-44.1 =73.5 m