• TwilightKiddy@programming.dev
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    14 hours ago

    The divisability rule for 7 is that the difference of doubled last digit of a number and the remaining part of that number is divisible by 7.

    E.g. 299’999 → 29’999 - 18 = 29’981 → 2’998 - 2 = 2’996 → 299 - 12 = 287 → 28 - 14 = 14 → 14 mod 7 = 0.

    It’s a very nasty divisibility rule. The one for 13 works in the same way, but instead of multiplying by 2, you multiply by 4. There are actually a couple of well-known rules for that, but these are the easiest to remember IMO.

    • darkpanda@lemmy.ca
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      13 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?