Answer
Rate of change=50
The amount saved is increasing at a rate of $50 per month.
Work Step by Step
To calculate the rate of change between two points $(a,f(a))$ and $(b,f(b))$, where $b \gt a$, we use the formula:
rate of change$=\frac{f(b)-f(a)}{b-a}$
We use the two points $(0,0)$ and $(4,200)$:
rate of change$=\frac{200-0}{4-0}=200/4=50$
Thus the amount saved must be increasing at a rate of $50 per month.
