7.1. A column of cars is moving along the highway at a speed of 80 km/h with a distance of 10 m between the cars. Passing a speed limit sign, all cars reduce their speed to 60 km/h. What will be the distance between the cars in the column after the sign?
Answer: 7.5 m. Solution. Let the speeds be $v_{1}=80$ km/h, $v_{2}=60$ km/h, and the distance $a=10$ m $=0.01$ km. The second car passes the sign later than the first by $\frac{a}{v_{1}}$ (hours). During this time, the first car will travel a distance of $v_{2} \cdot \frac{a}{v_{1}}=7.5 \text{ m}$, with this distance the cars will continue to move further.