#### Answer

The value of the machine is decreasing, on average, by $-\$4000/year$.

#### Work Step by Step

To calculate the average rate of change between two points $(x_1,y_1)$ and $(x_2,y_2)$, we use the formula:
$\displaystyle average~rate~of~change=\frac{y_2-y_1}{x_2-x_1}$
We calculate the average rate of change with the points $(0,20)$ and $(4,\ 4)$:
average rate of change$=\frac{4-20}{4-0}=\frac{-16}{4}=-4$ thousand/year
Therefore, the value of the machine is decreasing, on average, by $-\$4000/year$.