Two drivers travel from city A to city B and immediately return to city A. The first driver travels at a constant speed of $80 \mathrm{~km} / \mathrm{h}$, both ways. The second driver travels to city B at a constant speed of $90 \mathrm{~km} / \mathrm{h}$ and returns at a constant speed of $70 \mathrm{~km} / \mathrm{h}$. Which of these drivers spends less time on the round trip?
We know that speed is the ratio of the distance traveled to the time spent. Let $d$ be the distance between the two cities $\mathrm{A}$ and B.

- The first driver travels the distance of $2 d$ at a constant speed of $80 \mathrm{~km} / \mathrm{h}$, so the total time spent by this driver is

$$
t=\frac{2 d}{80}=\frac{d}{40} \text { hours. }
$$

- The second driver travels the distance $d$ on the way there at a constant speed of 90 $\mathrm{km} / \mathrm{h}$ and, on the way back, travels the same distance $d$ at a constant speed of $70 \mathrm{~km} / \mathrm{h}$. Therefore, the time spent on the round trip is

$$
t=\frac{d}{70}+\frac{d}{90}=\frac{16 d}{630}=\frac{8 d}{315} \text { hours. }
$$

Since

$$
\frac{d}{40}=\frac{8 d}{320}<\frac{8 d}{315}
$$

we verify that the driver traveling at a constant speed of $80 \mathrm{~km} / \mathrm{h}$ is the one who spends the least time on the round trip.