A balloon with mass 'm' is descending down with an acceleration 'a' (where a<g). How much mass should be removed from it so that it starts moving up with an acceleration 'a'?

Let F be the upthrust on the balloon.
When the balloon is descending with acceleration 'a', we have:
mg - F = ma (1)

When mass m' is removed, the balloon moves upwards with acceleration 'a'.
Then, F - (m - m')g = (m - m')a
From (1), F = mg - ma
Substituting this into the above equation:
mg - ma - (m - m')g = (m - m')a
mg - ma - mg + m'g = ma - m'a
m'g + m'a = 2ma
m'(g + a) = 2ma
m' = 2ma/(g + a)