• Karyoplasma
      3·
      1 year ago

      For divisibility by 13, take the last number, multiply by 4 and add to the rest.

      For divisibility by 17, take the last number, multiply by 5 and subtract from the rest.

      For divisibility by 19, take the last number, multiply by 2 and add to the rest.

      In fact, you can adapt the method to check for divisibility by any prime number k.