The volume of the balloon will halve
Explanation:
Boyle's law states that for an ideal gas kept at constant temperature, the pressure of the gas is proportional to its volume. Mathematically,

where
p is the gas pressure
V is the volume
The equation can also be rewritten as

And if we apply it to the gas inside the balloon in this problem (assuming its temperature is constant), we have:
is the initial pressure at sea level (the atmospheric pressure)
is the initial volume
is the final pressure
is the final volume
Substituting into the equation, we find:

Which means that the volume of the balloon will halve.
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