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AccountMaker@slrpnk.net ⁨2⁩ ⁨years⁩ ago

But what about 14, 21 and 28?

14 - 4*2 = 6, not divisible by 7

21 - 1*2 = 19, not divisible by 7

28 - 8*2 = 12, not divisible by 7

Or did I misunderstand the algorithm?

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Colalextrast@lemmynsfw.com ⁨2⁩ ⁨years⁩ ago
It goes like this

create 2 distinct numbers by isolating the last digit from the other. For example, 154 becomes 15 and 4.

double the number derived from the last digit. So, the four becomes 8.

3)subtract from the number derived from the preceeding digits. 15 - 8.

the resulting number is 7. Seven is divisible by 7, so we know 154 is divisible by 7.
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- AccountMaker@slrpnk.net ⁨2⁩ ⁨years⁩ ago
  Oooh, I didn’t realize that you subtract from the original number without the last digit. Thanks
  
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Kwdg@discuss.tchncs.de ⁨2⁩ ⁨years⁩ ago
Yeah you got it wrong, it’s 1 - 42 = -7 2 - 12 = 0 2 - 8*2 = -14

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