Matter is classified into three states: solid, liquid and gas. The behavior of gas is determined by pressure, volume and temperature. Changes in these variables can be calculated using Boyle's law, also called the combined gas law, where the starting and ending factors must equal in calculation. Pressure volume can be calculated using the combined gas law equation.
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Write the following formula and identify your variables:
P1 x V1 = (T1 x P2 x V2) / T2, where
P1 is the starting pressure.
V1 is the starting volume.
T1 is the starting temperature (measured in Kelvin).
P2 is the ending pressure.
V2 is the ending volume.
T2 is the ending temperature (measured in Kelvin). 
Insert the values of your known variables into the equation; for example:
P1 and V1 are unknown variables.
T1 = 300 deg. Kelvin (K)
P2 = 1 atmosphere (atm)
V2 = 1.5 L
T2 = 325 deg. Kelvin (K) 
Write the inserted values into the equation; for example:
P1 x V1 = (300 K x 1 atm x 1.5 L) / (325 K)
P1 x V1 = 450 K atm L / 325 K
P1 x V1 = 1.38 atm L
The starting volume pressure is 1.38 atm L. 
Assume that the starting pressure and temperature are lower than the ending pressure and temperature (i.e., the gas rises from below sea level to a hotter location at sea level); for example:
P1 = .75 atm
Reorganize the equation to find the starting volume; for example:
V1 = (T1 x P2 x V2) / (T2 x P1)
V1 = (300 K x 1 atm x 1.5 L) / (325 K x .75 atm)
V1 = (450 K atm L) / (243.75 K atm)
V1 = 1.85 L
The starting volume of gas would be 1.85 L. 
Assume that the starting volume is the same as the ending volume (i.e., what is the change in gas pressure); for example:
V1 = 1.5 L
Reorganize the equation to find the starting volume; for example:
P1 = (T1 x P2 x V2) / (T2 x V1)
P1 = (300 K x 1 atm x 1.5 L) / (325 K x .1.5 L)
P1 = (450 K atm L) / (487.5 K L)
P1 = .92 atm
The starting pressure of gas would be 0.92 atm, which is below sea level.