posted by Shoaib .
a ball is thrown vertically upwards with a velocity 'u' from the balloon descending with a constant velocity 'v'.. the ball will pass by the balloon after what time?
The answer will depend upon whether the velocity of the thrown ball is with respect to the ground or with respect to the balloon. After you have figured out which it is supposed to be, write equations for the height of the ball and the balloon relative to the altitude when the ball is thrown.
Set them equal and solve for t.