Partial Pressure 1

There are $10.0$ mol CO and $12.5$ mol O $_{2}$ in a container. The mixture is now heated, and CO $_{2}$ is formed by following reaction:

$\text{CO}(g) + \frac{1}{2} \text{O}_{2}(g) \rightarrow \text{CO}_{2}(g).$

What is the mole fraction of CO when $3.0$ mol CO $_{2}$ is present in the mixture?