assume the wooden piece prevents the balloon from rising, is not so heavy as to cause the balloon to descend. and the 15 m/s is horizontal velocity “riding the wind,” That horizontal velocity does not affect the time the wood will take to reach the ground after release. Initial vertical velocity is zero.
s = u t + 1/2 g t^2
s is the height above ground, 300 m.
u is initial vertical velocity, zero.
t is time to reach the ground.
g is acceleration of gravity near Earth, 9.8 m/s^2.
300 m = 0 t + 1/2 (9.8 m/s^2) t^2
300 m = (4.9 m/s^2) t^2
61.22 s^2 = t^2
7.82 seconds = t