How to calculate atmospheric composition
A really interesting problem, but I don't think you have enough information to solve it. At first glance, it seems like you would get one oxygen molecule in the atmosphere for every $\ce{CO2}$ molecule consumed. If we assume we are only making carbohydrates, then the process would be $\ce{CO2 + H2O -&gt; [CH2O] + O2}$. That would give a $70:20:5:5$ ratio of $\ce{CO2:O2:N2:Ar}$. However, plants will do more than just produce carbohydrates into their structures. They also produce fats, proteins, etc. Some of the $\ce{N2}$ would become incorporated. In addition, $\ce{CO2}$ is appreciably soluble in water (more so at lower temperatures). I think both factors would reduce the $\ce{CO2}$ and $\ce{N2}$ levels in the final mix.