Dalton's Law of partial pressures states that the gas pressure depends on the amount of gas particles in a given volume and at a given kinetic energy (temperature.) The total pressure is just the partial pressures added together:
Ptotal = P1 + P2 + P3 + …
Remember! Each sample of gas acts like there is no other sample in the mixture because gas pressure is caused by collisions of the gas particles on the sides of the container.