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

Mass is the amount of matter in an object.  weight is the measurement of the pull of gravity on an object.  say for example, you travel to the moon which has a different gravitational field compared to the gravitational field of earth. (gravitational fields: earth = 1, moon = 0.165)  now when you are on the moon, the pull of gravity on you is different than on the earth. and as weight is the measurement of the pull of gravity on an object, that means yes, your weight can and will change on the moon.  however, on the moon, on earth or even on jupiter, the amount of mass inside of you remains the same. meaning your physical mass remains constant.  in conclusion it is possible for your weight to change as your mass remains constant.