Why is 42 the answer to life the universe and everything and why is the question 6x9?

Answer 1

We will probably never know why 42 is the ultimate answer, because we will likely never know the question.

"What do you get when you multiply 6 by 9" is not the correct question. That was a corruption of the question, encoded in Arthur Dent's brain patterns. The complex biological computer program (also known as "the Earth") which had been calculating the question for millions of years got screwed up when the Golgafrinchans arrived on prehistoric Earth and destroyed the environment.

Answer 2

In Douglas Adams' book "The Hitchhiker's Guide to the Galaxy," the supercomputer Deep Thought is asked for the Answer to the Ultimate Question of Life, the Universe, and Everything. Deep Thought determines the answer is 42, but realizes it doesn't know what the actual question is. The joke about the question being "6 times 9" instead of the expected "6 times 7" is meant to highlight the absurdity and randomness of life.