Answer
Rate of change=-4
The value of the machine must be decreasing at a rate of $4000 per year.
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 $(0,20)$ and $(4,4)$:
rate of change$=\frac{4-20}{4-0}=\frac{-16}{4}=-4$
Thus the value of the machine must be decreasing at a rate of $4000 per year.
