Need help with the alveolar air equation

Ok guys, I need some help with the alveolar air equation:

I know it goes something like PAO2 = PIO2 - (PACO2/R) + F.


Scenario is: pt breathing room air @ sea level, PaO2 = 100, PaCO2 = 40. If you double the alveolar ventialtion for 10 minutes, what is the new PO2 & PCO2 assuming R=0.8?

Answer is PO2 = 125 & PCO2 = 20


Rationale is:
- increase alveolar ventilation --> increase PA02
- double alveolar ventilation --> no double in PAO2 in person whose PA)2 is already 104 b/c highest PAO2 one could possibly achieve at sea level


First of all, I can't see how the above #'s go into this equation. Second, I though that if you double ventilation, you can double O2 up to 150... But I may have it all wrong, everything's starting to look blurry to me.

Thanks in advance if anyone could enlighten me!!!
- Becky