1. **State the problem:** We need to find the average rate of change of the water depth $W$ between $x=100$ days and $x=300$ days. 2. **Formula:** The average rate of change of a function $W(x)$ between $x=a$ and $x=b$ is given by: $$\text{Average rate of change} = \frac{W(b) - W(a)}{b - a}$$ 3. **Identify values from the graph:** - At $x=100$, $W(100) \approx 90$ ft (peak near 150, so at 100 it is close to 90) - At $x=300$, $W(300) \approx 30$ ft (low point) 4. **Calculate the average rate of change:** $$\frac{W(300) - W(100)}{300 - 100} = \frac{30 - 90}{200} = \frac{-60}{200}$$ 5. **Simplify the fraction:** $$\frac{-60}{200} = \frac{\cancel{-60}}{\cancel{200}} = \frac{-3}{10} = -0.3$$ 6. **Interpretation:** The average rate of change is $-0.3$ ft/day, meaning the water depth decreased on average by 0.3 feet per day between day 100 and day 300. **Final answer:** $$\boxed{-0.3 \text{ ft/day}}$$