Answer
$30$ miles per hour
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 $[0.1,0.2]$ and the table,
a=$0.1,\ \quad D(0.1)=3$
b=$0.2,\ \quad D(0.2)=6$
$\displaystyle \frac{D(b)-D(a)}{b-a} =\displaystyle \frac{6-3}{0.2-0.1}$
$=\displaystyle \frac{3}{0.1}=30$ (miles per hour)
