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!
The reason, or at least a significant part of the reason, if not the complete reason, that Arthur derives that question in particular is because he's a result of the corruption of the calculation caused by the arrival of the B Ark of Golgafrinchams on Earth.
If Arthur had actually been a six foot tall Earth-ape descendant as described by the guide, his brain would have contained the actual answer rather than something almost but not quite totally unlike the true question.
But then the universe would have popped out of existence and been replaced by a more complicated one.
Again.
So we got lucky there.
Spot on.
Also: Knowledge of the answer precludes knowledge the question. Peak knew that.