Answer
$-200$ items per dollar
Work Step by Step
The average rate of change of $f(x)$ over the interval $[a, b]$ is $\displaystyle \frac{f(b)-f(a)}{b-a}$
Units: "(unit of f(x) ) per (unit of x)"
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Given the interval $[5,5.5]$ and the table,
a=$5,\ \quad q(5)=400$
b=$5.5,\ \quad q(5.5)=300$
$\displaystyle \frac{q(b)-q(a)}{b-a} =\displaystyle \frac{300-400}{5.5-5}$
$=\displaystyle \frac{-100}{0.5}=-200$ (items per dollar)
