Nearly all science fiction fans share a meme about the number 42. This meme comes from the Douglas Adams book, The Hitchhiker’s Guide to the Galaxy, one of the great “modern classics” (an apparent oxymoron, but it is just shorthand for ‘a recent work that is so good that someday it will be counted among the classics’). The book is the first in the “increasingly misnamed” trilogy that shares its name.
The trilogy is “increasingly misnamed” in that it now has five books. The joke is that, in science fiction, trilogies are as common as aliens, spaceships and time travel. In fact, depending on the context, there are a two trilogies that have earned the sobriquet, “The Trilogy.” (Issac Asimov‘s Foundation series in the context of pure SF; and, of course, J.R.R. Tolkien‘s Lord of the Rings books in the context of SF + fantasy.)
In any event, the number, 42, is the answer to the question.