The Answer is 42 - but what about the Question?

[Ouch!]

The only time the Question is actually expressed in the form of a question (when Arthur pulls pieces randomly out a scrabble jar), it says: "What do you get when you multiply six by nine?" This is not a typo. Six times nine equals 42 in base thirteen.

Adams denies that this joke was intentional, however. It's defunct, anyway, as Golgafrinchans landed on Earth (the computer calculating the Question) and replaced the inhabitants that were part of the program. As a descendent of Golgafrinchans, the question Arthus produces cannot be correct.

"The answer to this is very simple. It was a joke. It had to be a number, an ordinary, smallish number, and I chose that one. Binary representations, base thirteen, Tibetan monks are all complete nonsense. I sat at my desk, stared into the garden and thought '42 will do.' I typed it out. End of story."

Further, in the final novel, Ford tells the cab driver "just there, number forty-two... Right there!" in reference to club Beta. Club Beta, owned by Stavro Muller, was referred to long ago, in the second book, I believe. As long as Arthur had not yet visited Starvomullabeta (or something like that which sounded an awful lot like club Beta and its owner's name), he knew that he could not die.

There's other theories, but they're not worth arguing.

Edit: I wouldn't take anything in the book seriously or expect that it has any profound meaning. In the foreword Adams writes for The Ultimate Hitchhiker's Guide (a compliation of all five books and a short story detailing how Zaphod met Martin), he states that the idea came to him while hitchhiking across Europe. It's been a while since I read the foreword, but I think Adams was also drunk when the idea of a novel came to him. He didn't write it for many years after, anyway. The point is, I think looking for any sort of depth or meaning is a horrible waste of time.