How to calculate the relationship between the pressures of zinc, carbon monoxide, and carbon dioxide based on mole balance?
First of all, let me explain why your approach is a bit wrong...
Addition of steps will only give you the net reaction, which is what you got. But in the process, the mechanism becomes blurred. For example, the net reaction which you have expressed, doesn't tell us the full story. One completely overlooks the fact the CO is also recombining to give CO2, and that is exactly where the fallacy arises. 



My solution to the problem. Hope it helps.