The gas laws
Pressure, volume and temperature trade off through PV/T staying constant, and temperature must always be in kelvin.
A worked example
A 3.0 L balloon at 200 kPa and 280 K, moved to 100 kPa and 350 K, becomes (200 x 3.0 x 350) / (280 x 100) = 7.5 L.
The mistake people actually make
Convert degrees Celsius to kelvin by adding 273.15 before any gas calculation. Alberta's STP is 0 degrees C and 101.325 kPa, giving 22.4 L/mol, while SATP is 25 degrees C and 100 kPa, giving 24.8 L/mol.