A balloon is descending with a constant acceleration ‘a’ less than acceleration due to gravity. The mass of the balloon, with its basket and contents, is M. What mass m, of ballast(sand bags) should be released so that the balloon will begin to accelerate upward with constant acceleration ‘a’?

Boyant(B) forces in both the cases will be same.(as the volume wont change)
so in case 1
Mg-B=Ma
in case 2
B-(M-m)g=(M-m)a
..
obtain the value of B from eq. 1 and plug it into eq.2
you will get the answer i guess

Boyant(B) forces in both the cases will be same.(as the volume wont change)

so in case 1

Mg-B=Ma

in case 2

B-(M-m)g=(M-m)a

..

obtain the value of B from eq. 1 and plug it into eq.2

you will get the answer i guess

Thanks Sagar