6. Tire economy. The tires on the rear wheels of a truck wear out after 15,000 km of driving, and on the front wheels - after 25,000 km. How many kilometers can the truck travel without replacing the tires, if the front and rear tires are swapped at the right moment?
6. Let's consider what happens with one pair of tires. For every kilometer driven, the front tire wears out by $\frac{1}{25000}$, and the rear tire by $\frac{1}{15000}$. The maximum distance the car can travel with this set of tires can be found from the equation: $x\left(\frac{1}{25000}+\frac{1}{15000}\right)=2$. Answer: $18750 \mathrm{km}$.