The principle that the pressure exerted by a mixture of gases in a fixed volume is equal to the sum of the pressures that each gas would exert if it occupied the whole volume also called Dalton's Law of Partial Pressure.
P = Pnitrogen + Pcarbon dioxide + Poxygen
150 kPa = 100 kPa + 24 kPa + Poxygen
Poxygen = 150 kPa - 100 kPa - 24 kPa
Poxygen = 26 kPa