Dalton's law

The total pressure exerted by a gaseous mixture is equal to the sum of the pressures that would be exerted by the gases if they alone were present and occupied the total volume.

In mathematics this would mean the pressure P of a mixture of n gases can be defined as the summation

P = p(1) + p(2) + ... + p(n)

This means that each gas in a mixture acts as if the other gas is not present and that the pressures that come from each gas can simply be added. It is assumed that the gases do not react with each other.