Eight numbers emerge in sequence according to a certain system. One number is unknown. Can you figure out what it should be?
Each number is the sum of the squares of the digits of its predecessor.
42 = 16 → 12 + 62 = 37 → 32 + 72 = 58 → 52 + 82 = 89 → 82 + 92 = 145 → 12 + 42 + 52 = 42 → 42 + 22 = 20 → 22 + 02 = 4
The missing number is 20.
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This puzzle originally appeared in Spektrum der Wissenschaft and was reproduced with permission.