42

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In Douglas Adams' Hitchhiker Guide to the Galaxy, the answer to the ultimate question of the universe was 42. BigThink.com had an article today listing 5 actual questions of science that have the answer 42: 42 really is the answer to these 5 fundamental questions - Big Think.

More on Adams' classic: The Hitchhiker's Guide to the Galaxy - Wikipedia.

Update, 1/10/2024: Also see https://www.iflscience.com/life-the-universe-and-everything-why-42-really-is-the-ultimate-answer-72379 for additional questions answered with 42.