The total pressure of any system of gas molecules is equal to the pressure exerted by each an individual gas molecule. So p one plus p two plus all the way up to PM, assuming that my system is composed of M gas molecules. So this is simply a mathematical way of representing any series. So let's look at this system. So suppose we have a closed system of five gas molecules, two blue and three red. Now, we have two types of molecules and a total of five molecules.