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Panik (i.postimg.cc)
submitted 2 years ago by to c/lemmyshitpost@lemmy.world
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[–] 3 points 2 years ago* (last edited 2 years ago) (2 children)

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?

EDIT: I didn't realize that you remove the last digit when subtracting, got corrected in the replies.

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  • [–] 7 points 2 years ago* (1 child)

    It goes like this

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

    2. 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.

    4. the resulting number is 7. Seven is divisible by 7, so we know 154 is divisible by 7.

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