When combined, two gases have a pressure of 4.0 atm. if one gas has a pressure of 1.5 atm, what is the pressure of the second gas?

Pressure is additive. In a mixture of gases, the sum of the partial pressures is equal to the total pressure of the system. 
Partial pressure is the pressure exerted by individual gases.
in this case the two gases have a pressure of 4.0 atm.
this pressure is the sum of the partial pressures of the 2 gases making up the mixture.
4.0 atm = Partial pressure of gas A + partial pressure of gas B
partial pressure of A - 1.5 atm 
4.0 = 1.5 + partial pressure of gas B
4.0  - 1.5 = 2.5
pressure exerted by second gas is 2.5 atm

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Rn in the right answer