2. Ingots of highly enriched uranium must be stored in such a way that the distance to the nearest ingot is as large as possible. If there are several suitable places, any one of them can be chosen. Unfortunately, no one knows how many ingots will need to be stored. Ingots arrive one by one. Once an ingot is placed in storage, it cannot be moved.

An empty storage complex has 81 storage rooms, arranged in a single row. The first arriving ingot is placed in room 1, and the second, according to recommendations, is placed in room 81.

In which room can the 6th arriving ingot end up? List all possible answers.
Answer: $11, 31, 51$ or 71.