Stavro Mueller Beta - The question to the ultimate answer
We all know the question to the "Ultimate answer of Life, the Universe, and Everything" is forty-two, however we also know from other HHGTTG books that it is impossible to know both the answer and the question at the same time.
When Arthur Dent walkes into Stavro Mueller Beta, the HHGTTG universe ends (at least for Arthur Dent), and we find out that the new Guide has caused this to happen. We can therefore assume that the question happens sometime prior to the end.
If you listen carefully, when Ford Prefect is on his way to the club, he yells out to the cab driver "There, number forty-two!". Shortly afterwords we see the universe come to an end.
So, it is safe to assume that the universe was created simple for Arthur Dent and Tricia McMillian to end up back at earth at a time for them to get destroyed (this was not outside of the capabilities of the new Guide). So the answer is 'forty-two', the question is 'Where is Stavro Mueller Beta?'