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The Nines Trick
why digit sums reveal remainders
Add up the digits of a number: for 12,345, that's 1 plus 2 plus 3 plus 4 plus 5, which is 15. Divide 12,345 by 9 and the remainder is 6. Divide 15 by 9 and the remainder is also 6. Why?
Source: A classic puzzle, retold by INFINITE SUM and fact-checked against our curated notes
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