8.2. In the bus, there are single and double seats. In the morning, 13 people were sitting in the bus, and there were 9 completely free seats. In the evening, 10 people were sitting in the bus, and 6 seats were completely free. How many seats are there in the bus?
Answer: 16.

Solution: If in the morning passengers sat on 6 double seats (i.e., as densely as possible), then they occupied 7 seats, with 9 seats still free. In total: 16 seats. If they did not sit as densely, then they would have occupied more seats. That is, there are no fewer than 16 seats in the bus, on the one hand.

On the other hand, if in the evening each passenger took a seat, then 10 seats were occupied, with 6 still free, a total of 16 seats. If some sat together, there would have been fewer seats. Therefore, there are no more than 16 seats.

Thus, there are no more, but also no fewer than 16 seats. Therefore, there are exactly 16 seats.

Criteria: only the answer, answer with verification - 1 point, estimation from only one side - 3 points, no points deducted for the lack of an example.