Problem 7. Aся, Borya, Vasilina, and Grisha bought tickets to the cinema for one row. It is known that:

- There are a total of 9 seats in the row, numbered from 1 to 9.
- Borya did not sit in seat 4 or 6.
- Aся sat next to Vasilina and Grisha, and no one sat next to Borya.
- There were no more than two seats between Aся and Borya.

In which seat, with what number, was someone definitely sitting?

Answer: 5.
Solution. Note that Asey, Vasilina, and Grisha occupy three seats in a row, and Borya sits one seat away from them. Let's seat another child, Dima, in the free seat. Then, 5 children sit in a row. Thus, someone is sitting in the central seat of the row (that is, seat number 5). For any other seat, we can come up with an arrangement such that it is not occupied.

What if Dima is sitting in the 5th seat? Then Borya (since they are sitting next to each other) must sit either in seat 4 or 6, but this contradicts the condition. Therefore, in seat 5, either Asey, Borya, Vasilina, or Grisha is sitting.