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42 is the "Answer to the Ultimate Question of Life, the Universe, and Everything" in The Hitchhiker's Guide to the Galaxy books.
It takes Deep Thought 7½ million years to compute and check the answer, which turns out to be 42. Deep Thought points out that the answer seems meaningless because the beings who instructed it never actually knew what the Question was.
When asked to produce The Ultimate Question, Deep Thought says that it cannot. It can help to design an even more powerful computer, which can. This new computer will incorporate living beings into the "computational matrix" and will run for ten million years. It is the planet Earth.