13.409 A self-propelled roller can compact a strip 0.85 m wide, with each subsequent strip overlapping the previous one by $1 / 4$ of its width. At what speed should this roller move to ensure that a highway section 750 m long and 6.5 m wide can be compacted twice within a time not exceeding 6 hours and not less than 5 hours?
Solution. According to the condition, one pass of the roller over the highway section adds a strip of width $\frac{3}{4} \cdot 0.85=0.6375$ m, i.e., 10 passes will be sufficient to compact the entire strip. In the end, the roller will need to pass 20 times the entire section, i.e., 15 km.

From this, if $x$ is its speed, then $\frac{15}{6} \leq x \leq \frac{15}{5} \Rightarrow 2.5 \leq x \leq 3$.

Answer: from 2.5 to 3 km/h.