A gas sample is prepared in which the components have the following partial pressures: nitrogen, 555 mmHg; oxygen, 149 mmHg; water vapor, 13 mmHg; argon, 7 mmHg. What is the total pressure of this mixture?
Dalton's Law of Partial Pressure.
The law explains that a gas in a mixture exerts its own pressure independent of any other gas (if non-reactive gases) and the total pressure is the sum of the individual pressures.
Here, you are given the gases and the pressures they exert. To find the total pressure, you add all the individual pressures together.