Disclaimer- I do NOT want answers to the specific question. I want to understand how to get the answers and which formulae to use. If your comment doesn't contain some form of process I will delete it as it is against the guidelines to give or ask for answers directly. HOWEVER! I'm not sure on these calculations at all and I've probably made a lot of mistakes. So, I'd appreciate it if someone could check it over and let me know where I've gone wrong and give me tips to correct it.

The question is:
An athlete exhales 6.4g of breath at standard ambient temperature and pressure (25°C and 1atm), which is made up of 5% CO2, 16% O2 and 79% N2 by volume.
Calculate the following:
A) the molar mass of the mixture (kg/kmol)
B) the specific gas constant of the mixture (kJ/kg•K)
C) the mass fraction of O2
D) the volume of gas exhaled