9-1. Postman Pechkin calculated that he walked half the distance (at a speed of 5 km/h) and only a third of the time he was cycling (at a speed of 12 km/h). Did he make a mistake in his calculations?
Answer. Mistaken.

Solution. Let's denote the entire distance Pechkin traveled as $2 S$ km. Then, on foot, he covered a distance of $S$ km and spent $S / 5 = 0.2 S$ (hours) on it. According to the problem, this constituted $2 / 3$ of the total time spent, meaning the entire journey took $0.2 S : 2 / 3 = 0.3 S$ (hours), and he cycled for $0.1 S$ hours. Therefore, his speed should be $S / (0.1 S) = 10$ (km/h).

Criteria. Only the answer - 0 points. Complete solution - 7 points.