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A comment on Divisibility by 7 is a Walk on a Graph (2009). Parent comment

Re:Divisibility by 7 is a Walk on a Graph (2009) by chadzawistowski on Monday November 09, 2015 @12:06AM (#10531486)
Since you scratched off the last digit, 10 is really 100, plus 5 for the digit you scratched off = 105.

105 is a multiple of 21.

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