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The number must meet two criteria:

  • Take the original number excluding its last digit and subtract two times the last digit from it. The result must be divisible by 7.
  • Take the original number excluding its last digit and subtract nine times the last digit from it. The result must be divisible by 13.

In both cases, you can repeat the "truncate-and-subtract multiple" stage several times, if required.

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7y ago

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