Explanation :
The forces acting on hot- air balloon are:
Weight, (W)
Force due to air resistance, (F)
Upthrust force, (U)
Its weight W is acting in downward direction. The upthrust force U acts in upward direction. When the balloon is moving upward, the air resistance is in downward and vice versa.
In this case, the hot-air balloon descends vertically at constant speed.
so, 
and 
so,
....................(1)
when it is ascending let the weight that it is releasing is R, so
..........(2)
solving equation (1) and (2)

2F is the weight of material that must be released from the balloon so that it ascends vertically at the same constant speed.
Answer:
hydrogen chloride...........
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Answer:
-0.0047 rad/s²
335.103 seconds
99.18 seconds
Explanation:
= Final angular velocity
= Initial angular velocity = 1.5 ra/s
= Angular acceleration
= Angle of rotation = 40 rev
t = Time taken
Equation of rotational motion

Acceleration while slowing down is -0.0047 rad/s²

Time taken to slow down is 335.103 seconds

Solving the equation

The time required for it to complete the first 20 is 99.18 seconds as 539.11>335.103