transcript:
cat mum: what's 6 times 7?
cat boy (making faces): 6 7!!!
cat mum: hilarious, as always. but actually ...?
cat boy: 42
cat mum: 6 x 7 - 6 7 - is 42?!
cat boy: yeah ... so?
cat mum (making faces): that means 6 7 is the answer to the ultimate question of life, the universe and everything!
cat boy: daddy! mommy's being weird again!
cat dad: aww, you see son, - your mommy is just a huge dork!
cat mum: history doesn't repeat itself, but it rhymes!
I think they find out in book 3 or 4 that that's the question, but it's been years since I've read them.
Sort of.
The mice decide that they'll just choose the question "how many roads must a man walk down?" and be done with it.
Arthur divines that the question is "what is six by nine" but that gives the wrong answer, because the question itself is wrong.
Wasn't there a part in a later book, where they find the question on prehistoric earth?
He was on prehistoric earth when he figured out that he could try to "print out" the answer since he was part of the program and got "what is six by nine", so that may be what you're thinking of