Combined gas law problem: a balloon is filled with 500.0 ml of helium at a temperature of 27 degrees celsius and 755 mmhg. as the balloon rises in the atmosphere, the pressure and temperature drop. what volume will it have when it reaches an altitude where the temperature is -33 degrees celsius and the pressure is 0.65 atm?

The statement of the combined gas law for a fixed amount of gas is,
PV/T = constant
Here, the units of pressure and volume must be consistent and the temperature must be the absolute temperature (Kelvin or Rankine).
0.65 atm is equivalent to 494 mmHg
Using the equation:
(755 x 500) / (27 + 273) = (494 x V) / (-33 + 273)
V = 3396 ml = 3.4 liters