Using OLS, we regress $y$ onto $X_1$ and find that the model has an $R^2$ of 0.15. We also regress $y$ onto $X_2$ but this time the model has an $R^2$ of 0.2. Let $[\text{min}, \text{max}]$ denote the lower and upper-bound of the $R^2$ of a model which regresses $y$ onto $X_1, X_2$. Express your answer as $\frac{\text{max}}{\text{min}}$.