4. Car 204 transports daily necessities from Town A to Village B, passing through 20 kilometers of uphill road, 14 kilometers of downhill road, and 5 kilometers of flat road. It then transports grain from Village B back to Town A. The time difference for the round trip is 10 minutes. It is known that the speed ratio of the car when going uphill, downhill, and on flat roads is 3:6:5. Try to find:
(1) The average speed of the car when going uphill, downhill, and on flat roads,
(2) The time required for the car to travel from Town A to Village B, and from Village B to Town A.
[Solution] (1) According to the problem, let the average speeds of the car when going uphill, downhill, and on a flat road be $3v, 6v, 5v$ (km/h) respectively.
Since $\quad\left(\frac{20}{3 v}+\frac{14}{6 v}+\frac{5}{5 v}\right)-\left(\frac{20}{6 v}+\frac{14}{3 v}+\frac{5}{5 v}\right)=\frac{1}{6}$,
that is, $\frac{6}{3 v}-\frac{6}{6 v}=\frac{1}{6}$, hence $v=6$.
Therefore, the average speeds when going uphill, downhill, and on a flat road are $18, 36, 30$ (km/h) respectively.
(2) The time required for Town A to Village B is:
$$\begin{aligned}
& \frac{20}{18}+\frac{14}{36}+\frac{5}{30}=\frac{1}{18}(20+7+3) \\
= & \frac{30}{18}=1 \frac{2}{3} \text { (hours). }
\end{aligned}$$

The time required for Village B to Town A is
$$1 \frac{2}{3}-\frac{1}{6}=1 \frac{1}{2} \text { (hours). }$$