Answer
$-\$ 2.33 $ per month
Work Step by Step
The average rate of change of $f(x)$ over the interval $[a, b]$ is $\displaystyle \frac{f(b)-f(a)}{b-a}$
Units: "(unit of f(x) ) per (unit of x)"
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Given the interval $[2,5]$ and the graph,
a=$1,\ \quad $(1,28) on the graph $\Rightarrow f(1)=28$
b=$5,\ \quad $(5,21) on the graph $\Rightarrow f(5)=21$
$\displaystyle \frac{f(b)-f(a)}{b-a} =\displaystyle \frac{21-28}{5-1}=\frac{-7}{4}$ (dollars per month)
$\approx-\$ 2.33 $per month
