Mean of digits
Problem statement
What number equals the average of its digits?
Comment on metapuzzle: show
Solution to puzzle: show
The intended answer is "4.5". This is a convincing answer, and is the intended one.
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I liked this puzzle, because it is so clearly the intended one, and because I failed to think of it.
The best answer I could think of was "undefined", or for a computer person "UNDEF". It contains no digits, so when you divide the sum of its digits, 0, by the count of its digits, 0, you get 0/0, which is undefined.
Source: Alex Bellos, in the Guardian, 2025-04-13
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