This is another question that raised howls of anguish in the quiz I took part in, when the answer was read out. And rather more understandably than the first (Question 34)! Indeed, it has to be said that anyone who sets this question is playing with fire. You surely have to be a real Douglas Adams fan to know this answer.
Except that this is not actually the Ultimate Question at all.
Somewhere in the "increasingly inaccurately named trilogy", it is explained that the question became corrupted because the Golgafrinchans "mucked up".
The Golgafrinchans were a race of humanoid beings who split their population into three distinct groups, and sent their third group, the middlemen – the ones that weren't thinkers (leaders) or doers (workers) – on a spaceship, which eventually ended up on planet Earth. These Golgafrinchan middle men then wiped out the entire population of Earth.
Earth is in fact a computer, built from organic components to work out the ultimate question of life, the Universe and everything (after Deep Thought had worked out the answer but without knowing the question). After the Golgafrinchans wiped out the original population of Earth, they repopulated it with their own descendants. This constituted a major input error, meaning that the program could not function correctly and the question could never be derived. (Or something like that; I'm extrapolating from various sources, including this one and this one.)
Another website, called Mathematical Fiction, has a slightly different explanation. This answer (or rather, this question) "is later discounted when it is realized that the universe was just a fake one made for Zaphod Beeblebrox. In fact, it is claimed in the book that the question and answer cannot both be known in the same universe (a sort of "Uncertainty Principle").
"Also" (it continues), "it should be pointed out that 6 x 9 = 42 is in fact true ... in base 13."
Douglas Adams later claimed (and I for one believe him) that he hadn't known this. In his last television interview (you can watch it here on YouTube), he explained that "the answer to the ultimate question of life, the Universe and everything, is 42. And the big issue is therefore: What is the question? All I can say is that in a quantum universe, it's impossible to know both the question and the answer."
So there you have it: because Earth was corrupted by the Golgafrinchans, the question it came up with ("What do you get if you multiply six by nine?") is not the correct one. And in fact, since we know the answer, we can never know the question. So this question should never be asked in a quiz.
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