Chapter 2, Functions - Section 2.4 - Average Rate of Change of a Function - 2.4 Exercises - Page 224: 29

$-\frac{1}{4}$ ft/day

We see from the graph that $W(200)=50$ and $W(100)=75$. We use this information to find the average rate of change for this interval: $\displaystyle \frac{W(200)-W(100)}{200-100}=\frac{50-75}{200-100}=\frac{-25}{100}=-\frac{1}{4}$ ft/day