I figured this one out ages ago. It's in base 13.
According to the books, the ultimate question to the answer is "What do you get when you multiply 6 by 9?", to which Arthur Dent says, "I always thought there was something fundamentally wrong with the universe".
But 6 time 9 is 54, which in base 13 is 42 ...
Yeah, I know. I have too much time on my hands :rofl