Wild About Math! Making Math fun and accessible

22Nov/111

More fun with the number 11

In honor of 11/22/11 (22 = 11+11), here is a nice complement to my 11/11/11 post.

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  1. A number is divisible by 11 if this is true:
    1st Step: Starting from the one’s digit add every other digit
    2nd Step: Add the remaining digits together
    3rd Step: Subtract 1st Step from the 2nd Step
    *If this value is 0 then the number is divisible by 11. If it is not 0 then this is the remainder after dividing by 11 if it is positive. If the number is negative add 11 to it to get the remainder.
    Ex [1] 6613585 is divisible by 11 since (5+5+1+6) – (8+3+6) = 0.
    Take another number, such as 365167484.
    Add the first, third, fifth, seventh,.., digits…..3 + 5 + 6 + 4 + 4 = 22
    Add the second, fourth, sixth, eighth,.., digits…..6 + 1 + 7 + 8 = 22
    If the difference, including 0, is divisible by 11, then so is the number.
    22 – 22 = 0 so 365167484 is evenly divisible by 11.


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