IF A BALLON WITH SAND IS COMING DOWN WITH UNIFORM VELOCITY.AFTER IT COMES DOWN HOW MUCH SAND SHOULD WE REMOVE FROM IT SO THAT IT GOES UP WITH UNIFORM VELOCITY?FORMULA?

The various forces on the balloon are 

1) weight of sand + balloon ( always downwards)

2) air resistance  F  ( opposite to direction of motion)

3) Buoyant force  B  ( always upwards )

Say initial mass of balloon is m_1.  So we can write

                       M₁g = B + F

Now say the final mass of the balloon after removing some sand be M_2.

     M₂g + F = B

so M₁ - M₂ = 2F /g