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?
Let $d$ be the distance between cities $\mathrm{A}$ and B, and remember that time $=$ distance $/$ speed.

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

$$
t=\frac{2 d}{80}=\frac{d}{40}
$$

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

$$
t^{\prime}=\frac{d}{70}+\frac{d}{90}=\frac{16 d}{630}=\frac{8 d}{315}
$$

Since

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

it is concluded 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.