A stone is dropped from a rising balloon at a height of 91.8 m above the ground and it reaches the ground in 6 s.
If the stone were dropped from a stationary balloon, the time t taken for it to reach the ground would have been given by 91.8 = (1/2)*9.8*t^2
=> t = sqrt (91.8/4.9)
=> t = 4.328 s
But the stone takes 6 s to reach the ground. If the upward velocity of the balloon in V.
91.8 = -V*6 + (1/2)*9.8*36
=> 6*V = -84.6
=> V = 14.1 m/s
The balloon is rising up at 14.1 m/s when the stone is dropped from it.