In a car race, a driver covered three sections: one of $240 \mathrm{~km}$, one of $300 \mathrm{~km}$, and one of $400 \mathrm{~km}$. The driver knows that the average speeds on these sections were $40 \mathrm{~km} / \mathrm{h}$, $75 \mathrm{~km} / \mathrm{h}$, and $80 \mathrm{~km} / \mathrm{h}$, but does not remember which of these speeds corresponds to each of these sections. We can guarantee that the total time in hours spent by the driver to cover the three sections was:

(a) less than or equal to 13 hours;
(b) greater than or equal to 13 hours and less than or equal to 16 hours;

(c) greater than or equal to 16 hours and less than or equal to 17 hours;

(d) greater than or equal to 15 hours and less than or equal to 18 hours;

(e) greater than or equal to 18 hours.
The correct option is (d).

The shortest travel time is achieved when the longest stretch is covered at the highest speed and the shortest stretch at the lowest speed. The longest time is achieved when the longest stretch is covered at the lowest speed and the shortest stretch at the highest speed. Thus, the total time spent by the driver on the three stretches is, at a minimum, $\frac{240}{40}+\frac{300}{75}+\frac{400}{80}=15$ hours and, at a maximum, $\frac{240}{800}+\frac{300}{75}+\frac{400}{400}=17$ hours.