• darkpanda@lemmy.ca
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    10 hours ago

    If all of the digits summed recursively reduce to a 9, then the number is divisible by 9 and also by 3.

    If the difference between the sums of alternating sets digits in a number is divisible by 11, then the number itself is divisible by 11.

    That’s all I can remember, but yay for math right?

    • TwilightKiddy@programming.dev
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      10 hours ago

      Well, on the side of easy ones there is “if the last digit is divisible by 2, whole number is divisible by 2”. Also works for 5. And if you take last 2 digits, it works for 4. And the legendary “if it ends with 0, it’s divisible by 10”.