1. **State the problem:** We are given a proportional relationship between the number of tokens Brenda uses, $x$, and the number of pitches thrown by the pitching machine, $y$. We need to find the constant of proportionality. 2. **Understand proportional relationships:** When two quantities are proportional, they satisfy the equation $$y = kx$$ where $k$ is the constant of proportionality. 3. **Use the given data:** The graph shows the line passes through the origin $(0,0)$ and the point $(1,10)$, meaning when $x=1$, $y=10$. 4. **Find the constant $k$:** Substitute $x=1$ and $y=10$ into the formula: $$10 = k \times 1$$ 5. **Solve for $k$:** $$k = \frac{10}{\cancel{1}} = 10$$ 6. **Interpretation:** The constant of proportionality is 10, meaning each token corresponds to 10 pitches. **Final answer:** $$\boxed{10}$$