#### Answer

The average rate of change in the interval $\$50$.
An average of $\$50$ is saved each month.

#### Work Step by Step

The graph is from $x=0$ to $x=4$ so the interval is $[0, 4]$.
RECALL:
The average rate of change in the interval $[a, b]$ is given by the formula:
average rate of change = $\dfrac{f(b)-f(a)}{b-a}$
The given graph has $f(0) = 0$ and $f(4) = 200$.
Thus, the average rate of change in the interval $[0, 4]$ is:
$=\dfrac{f(4)-f(0)}{4-0}
\\=\dfrac{200-0}{4}
\\=\dfrac{200}{4}
\\=50$
Therefore, an average of $\$50$ is saved each month.