Analyzing motion of bulletw.r.t balloon we usevBB=u−25 acc of bullet a=−g=−10 m/s2when bullet is fired at t = 5 s, the balloon is at a height of h = vt = 25x5 = 125 mu should be such that it will reach to a minimum height of 125 m⇒ 125=(u−25)22g⇒ u−25=125×2×10=50 m/s                                         u=75 m/s