8.1. Two cars are driving on a highway at a speed of 80 km/h and with an interval of 10 m. At the speed limit sign, the cars instantly reduce their speed to 60 km/h. What will be the interval between them after the speed limit sign?
Answer: $7.5 \mathrm{M}$.

Solution. Let $v$ (m/hour) be the speed of the cars before the sign, and $u$ (m/hour) be the speed of the cars after the sign. The second car will pass the sign later than the first by $10 / v$ (hours). During this time, the first car will travel $10 u / v$ (meters) $= 10 \cdot 6 / 8 = 7.5$ meters. This interval will be maintained after the sign. Comment. As can be seen from the solution, the result depends only on the ratio $u / v$, so it is not necessary to convert the speeds to other units of measurement.