A balloon with mass $m$ is descending down with an acceleration $a$ (where $a<g$). How much mass should be…

Let the upward force be $F$.

When balloon descending down, then  $mg-F=ma$ …(i)

Let the mass removed be $m\text{'}$.

When balloon ascending upward, then $F-\left(m-m^{\text{'}}\right)g=\left(m-m\text{'}\right)a$
from (i)
$F-mg+m^{\text{'}}g=ma-m\text{'}a$
$mg-ma-mg+m^{\text{'}}g=ma-m\text{'}a$
$m^{\text{'}}\left(g+a\right)=2ma$
$m^{\text{'}}=\frac{2ma}{g+a}$