I’m just going to check I am interpreting everything correctly.
385 cm? should read as 385 cm^3, 100 ka should read as 100 kPa and 2.89 × 102 mol should read as 2.89 × 10^-2 mol
Starting from pV = nRT, can you rearrange to make n the subject?
Converting 25°C to K and either p or V to appropriate units, can you now calculate a value for n?
Mole fraction of CO2 = (moles of CO2)/(moles of CO2 + moles of Ar) - does this give the final answer you are expecting?