Answer
The mean of these two symmetric difference quotients is
$-0.39375 \mathrm{kph}\times\mathrm{km} / \mathrm{car}$.
Work Step by Step
Let $S(q)$ be the function determining $S$ given $q$. The symmetric difference quotient with $h=10$ is
$$
S^{\prime}(80) \approx \frac{S(90)-S(70)}{20}=\frac{60-67.5}{20}=-0.375
$$
with $h=20$
$$
S^{\prime}(80) \approx \frac{S(100)-S(60)}{40}=\frac{56-72.5}{40}=-0.4125
$$
The mean of these two symmetric difference quotients is
$-0.39375 \mathrm{kph}\times\mathrm{km} / \mathrm{car}$.
