What's the answer to life, the universe, and everything?

The answer to the great Question of Life, the Universe, and Everything is . . . 42. 

That's the answer that Deep Thought gives at the end of chapter 27.  Phouchg and Loonquawl are obviously mystified and even a bit upset.  They can't believe that the answer to the ultimate question is a seemingly random number. 

”Forty-two!” yelled Loonquawl. ”Is that all you’ve got to show for seven and a half million years’ work?”

Deep Thought confirms that, yes, 42 is definitely the correct answer, but Deep Thought admits that he thinks the problem is that the initial question is incorrect.  

"I think the problem, to be quite honest with you, is that you’ve never actually known what the question is. . . So once you do know what the question actually is, you’ll know what the answer means.”

So Deep Thought builds a "machine" to answer the question of what is the actual real question.  He calls his machine "Earth."  Unfortunately with only 5 minutes to go before Earth reveals the question, the Vogons showed up and destroyed the planet.  Earth Two begins, and to make a confusing story more frustrating, the answer is still 42, but the question is still unknown.  Book ends.