What number should replace the question mark?
The number 129 replaces the question mark. The key is the sequence of prime numbers:
2, 3, 5, 7, 11, 13, 17, 19, 23, 29, …
The number in each polygon with n sides is the sum of the first n prime numbers’ values.
The following applies to the decagon:
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This puzzle originally appeared in Spektrum der Wissenschaft and was reproduced with permission.
