Yes, a simple consequence of e.g. 3-digit number being: 100x + 10y + z modulo 3 ... | Hacker News

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		DmitryOlshansky on April 22, 2020 \| parent \| context \| favorite \| on: Why are Soviet math textbooks so hardcore in compa... Yes, a simple consequence of e.g. 3-digit number being: 100x + 10y + z modulo 3 Removing 9y and 99x gives equivalence: x + y + z modulo 3 Now what’s left is induction with proper base (1-digit).

red_admiral on April 22, 2020 [–]

As dan-robertson mentioned in another branch, you don't even need induction - you can sidestep it by writing X = sum_i x_i 10^i and noticing that all the 10^i are 1 modulo 9 (and therefore modulo 3).

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