Final answer:
The speed of the balloon when it is empty is 15 m/s in the opposite direction.
Step-by-step explanation:
The speed of the balloon when it is empty can be calculated using the formula:
vf = vrel - v
Where vf is the final velocity of the balloon, vrel is the relative velocity with respect to the empty balloon, and v is the velocity of the balloon when it is empty.
Given that vrel = 15 m/s, we can substitute the values and solve for v:
vf = 0 - 15
v = -15 m/s
Therefore, the speed of the balloon when it is empty is 15 m/s in the opposite direction.