• Hjalmar@feddit.nu
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    1 day ago

    Il do it for disability by three and a three digit numbers with the digits a, b and c. The value of that number then is 100a + 10b + c. They concept is the same for nine.

    100a + 10b + a mod 3 =
    a + b + a
    

    This means that, mod 3, a three digit number is equivalent to the sum of it’s digits and therefore preserves disability by 3.