2. The front tire of a bicycle wears out after $250 \mathrm{~km}$, while the rear tire wears out after $150 \mathrm{~km}$. After how many kilometers should the tires be swapped so that they wear out simultaneously?
Solution. If the tires need to be rotated every $x \mathrm{~km}$, then according to the problem statement, we will have

$$
\frac{x}{250}+\frac{x}{150}=1
$$

from which we obtain $x=93.75 \mathrm{~km}$.