What is the partial pressure (in mmHg) of argon in a 5.00 L vessel that contains 0.757 mol of methane.....

What is the partial pressure (in mmHg) of argon in a 5.00 L vessel that contains 0.757 mol of methane, 0.210 mol of ethane and 0.810 moles of argon at a total pressure of 875 mm Hg?

(0.810)/(0.810+0.757+0.210) all of that, times 875

equals the partial pressure of argon in mmHg.

What you do is just multiply total pressure by the mole fraction of the gas you're looking at, and you get the partial pressure of that gas.