Answer:
15.33623 m/s
Step-by-step explanation:
u = Initial velocity
v = Final velocity
s = Displacement
a = Acceleration due to gravity = 9.8 m/s²

The initial velocity by which the balloon should be thrown up is 15.33623 m/s
At the maximum height the initial velocity becomes 0

Hence, the balloon will hit the ground at 15.33623 m/s