When combined two gases have a pressure of 4.0 atm. If one has a pressure of 1.5 atm, the pressure of the second gas will be; 4.0 atm- 1.5 atm = 2.5 atm.
Total pressure = Partial pressure of one gas + partial pressure of the second gas.Therefore; the pressure of the second gas will be; 2.5 atm