"Also, none of you actually compute 5x3 = 15. You just know the answer."

Yes, you are using a mental hash table, which have an expected access time complexity of O(1). GPT can also do this, it has already memorized a huge amount of data. However, ArcticWinter's statement remains correct: for non-O(1) questions that it has not yet memorized, it is IMPOSSIBLE for the current GPT versions to calculate the answer.

Of course, it can still guess and come pretty close, but it will never be certain of the answer.