101010 is 42 in binary

And 42 is, of course, the "Ultimate Answer to the Ultimate Question of Life, The Universe, and Everything."

It takes Deep Thought 7½ million years to compute and check the answer, which turns out to be 42. Unfortunately, The Ultimate Question itself is unknown.

When asked to produce The Ultimate Question, the computer says that it cannot; however, it can help to design an even more powerful computer (the Earth), that can. The programmers then embark on a further ten-million-year program to discover The Ultimate Question. This new computer will incorporate living beings in the "computational matrix", with the pan-dimensional creators assuming the form of mice. The process is hindered after eight million years by the unexpected arrival on Earth of the Golgafrinchans and then is ruined completely, five minutes before completion, when the Earth is destroyed by the Vogons to make way for a new Hyperspace Bypass...

At the end of the first radio series (and television series, as well as the novel The Restaurant at the End of the Universe) Arthur Dent, having escaped the Earth's destruction, potentially has some of the computational matrix in his brain. He attempts to discover The Ultimate Question by extracting it from his brainwave patterns, as abusively suggested by Marvin the Paranoid Android, when a Scrabble-playing caveman spells out forty two. Arthur pulls random letters from a bag, but only gets the sentence "What do you get if you multiply six by nine"?

Some readers subsequently noticed that 6₁₃ × 9₁₃ = 42₁₃ (using base 13). Douglas Adams later joked about this observation, saying, "I may be a sorry case, but I don't write jokes in base 13."

The number 42 also appears frequently in the work of Lewis Carroll, and some critics have suggested that this was an influence.  Other purported Carroll influences include that Adams named the episodes of the original radio series of The Hitchhiker's Guide to the Galaxy "fits", the word Carroll used to name the chapters of The Hunting of the Snark.