In other words, the pressure that this gas molecule exerts on the walls of my system of my container is independent of what this guy exerts on my wall of the container. So to find the total pressure, my system of two gas molecules, I simply have to add up this pressure and this pressure. And that's exactly what B states. 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.