Ans: (a) Part I (b) $8:26$ (c) $15\text{ m/s}$
- Note that the slope of the graph represents the average speed. Since the slope of Part I is the smallest among three parts, then the average speed of Part I is the lowest.
- Let $t$ hour be the time needed for travelling from $B$ to $C$.
$\begin{array}{rcl}
\dfrac{18-4}{t} & = & 56 \\
t & = & \dfrac{1}{4}
\end{array}$$\therefore $ John needs $15$ minutes travelling from $B$ to $C$. i.e. He is at $C$ at $8:26$.
- The average speed from $A$ to $D$
$\begin{array}{cl}
= & \dfrac{27-0}{0.5} \\
= & 54\mbox{ km/h} \\
= & 54\div 3.6\mbox{ m/s} \\
= & 15\mbox{ m/s}
\end{array}$