If the atmospheric pressure at 30.0 km is 28.0 torr and the temperature is -39.0 °c, what is the volume of the balloon when it bursts?

We from gas equation: 

PV=mRT

Where:
P is pressure (1 torr = 0.013N/cm2)
V is the volume of the balloon
m is the mass of gas in the balloon (constant)
R is universal gas constant divided by mean molar wt of air (about 28 g/mol) T is thermodynamic temperature (T in Kelvin; T=273 + t (in deg C)

So, For given values in gas equation and assuming we have 1 gm of gas in the balloon, we get: 

28 * V = 1 * R * (-39+273)
or, 28 * 0.013 * V = 1 * 28 * 324
or, V = 324 / 0.013 cm^3
or, V = 18000 cm^3

Independent variable is something that is changed in a experiment. 

Your answer is 32 hope i