Answer
The student's math SAT increases at an average of $0.80$ points for every ${{\$}} 1000 $ additional household income.
Work Step by Step
The units of the average rate of change of $f$ are units of $f(x)$ per (additional) unit of $x$.
When the household in which the student lives has an income which is between ${{\$}} 40,000$ and ${{\$}} 80,000$, the student's math SAT increases at an average of $0.80$ points for every ${{\$}} 1000 $ additional household income.
