Some students went on a field trip. At a speed of 6 km/h, they travelled by bikes for 2 hours; then at a speed of 2 km/h, they walked for another hour; then they took rest for an hour and had lunch for half an hour; then at a speed of 1 m/s, they walked for an hour to visit a garden; and finally at a speed 4 km/h, they returned home in 3 hours. Their average speed, in km/h, (rounded off to two places of decimal) is:

$$\eqalign{ & {\text{Total distance}} \cr & = 6 \times 2 + 2 \times 1 + \frac{{1 \times 18}}{5} \times 1 + 4 \times 3 \cr & = 12 + 2 + 12 + 3.6 \cr & = 29.6\,{\text{km}} \cr & {\text{Total time}} \cr & = 2 + 1 + 1 + \frac{1}{2} + 1 + 3 \cr & = 8\frac{1}{2} \cr & = \frac{{17}}{2}{\text{hr}}{\text{.}} \cr & {\text{Average speed}} \cr & = \frac{{29.6}}{{17}} \times 2 \cr & = \frac{{59.2}}{{17}} \cr & = 3.4823 \cr} $$