The combined gas law is a gas law which combines Charles's law, Boyle's law, and Gay-Lussac's law. Boyle's law states that pressure and volume are inversely proportional to each other at constant temperature. Charles's law states that volume and temperature are directly proportional to each other at a constant pressure. Gay-Lussac's law states that pressure and temperature are directly proportional to each other at a constant volume.
The inter-dependence of these variables is shown in the combined gas law, which clearly states that as the pressure increases, the volume decreases and the temperature increases, the volume increases. And The ratio between the pressure-volume product and the temperature of a system remains constant.

The combined gas law is expressed as: $\frac{\mathrm{PV}}{T}=k$
where:
P is the pressure of the gas.
V is the volume of the gas.
T is the temperature of the gas (measured in Kelvin).
k is a constant.

Because the formula is equal to a constant, Combined gas law can be rewritten as: $\frac{P_1{V}_1}{T_1}=\frac{P_2{V}_2}{T_2}$

Example:
4 liters of a gas in a tank has a pressure of 500 torr at 200 K. Calculate the temperature when the volume is 1 liter and the pressure is 1000 torr?

$\frac{P_1{V}_1}{T_1}=\frac{P_2{V}_2}{T_2}$

$T_2=T_{1}x\frac{P_2{V}_2}{P_1{V}_1}=200Kx\frac{1000x1}{500x4}=400K$

Combined gas law is based upon absolute temperature. The absolute temperature is calculated by adding 273 to the temperature in Celsius scale. 10°C is equiavalent to 283 K.

$T_{}=10\mathrm{°C}+273=283K$