#### Answer

(a) Average speed is $74.73\,\mathrm{km/h}$.
(b) Average velocity is zero.

#### Work Step by Step

(a) Total distance traveled, $d=300+300=600\,\mathrm{km}$
Total time taken,
$t=\frac{300}{75}+0.5+\frac{300}{85}=8.03\,\mathrm{h}$
Average speed,
\begin{align*}
s&=\frac{d}{t}=\frac{600}{8.03}\\&=74.73\,\mathrm{km/h}
\end{align*}(b) Since net displacement is zero, average velocity is zero.