Answer:
The calculated result is 11.7 ft
Explanation:
You can apply the combined gas law, which incorporates Boyle's law, Charles's law, and Gay-Lussac's Law, because hydrogen demonstrates ideal gas behavior under these specific conditions.

where the subscripts indicate "p" for pressure, "V" for volume, and "T" for temperature (in Kelvin) at varying moments. Let's denote
as the balloon at 150,000 ft so


and
.
Then
represents the point at which the balloon is on the ground.
and
.
Based on the first equation
, we find
and consequently the radius turns out to be
.