Easy Multiplication of 11s.

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It won’t work if your product is over a 1000 though. Sadly.

6 thoughts on “Easy Multiplication of 11s.”

  1. Blaise

    if the sum of the digits is 10 or more, you add one to the first digit of the original number and have the ones digit go in the middle. The last digit stays the same.

    48*11 has 4+8=12, so it’s 5_2_8 instead of 4_12_8 or 4_2_8. 528 is the product.

    with something like 95*11, it’s 9_14_5, so it’s 10_4_5, which is 1045.

    Once you get past the early hundreds, I don’t know how the pattern continues, and I don’t care enough to find out. That’s sad, I guess.

  2. Steven Swift

    Very well done on figuring this out for yourself! This is a well known result, and the trick for doing it for any number of digits is not very hard either. I’d tell u here how to do it, but….

    I encourage students to figure stuff out for themselves. Great that you did. I’m sorry you feel sad, but that is when you could reach out for help. Just google “multiply by eleven” and you’ll find the answer at many websites. There is a way. Given that you figured out the way for two digit numbers by yourself, you will easily understand the more “advanced” method.

    Good luck!

  4. EmotionalCalculator

    Here’s a neat trick (related to 11’s).

    Take four students – four boys, and four girls.
    Have them stand in a single row, side by side.
    (they must be in BOY-GIRL-BOY-GIRL order)
    Give them each a single paper with a single digit
    on it so the combined number (when read across) is
    “76389412” – each single digit being held by a
    “BGBGBGBG” (B==Boy,Girl==Girl)

    Notice this number is evenly divisible by 11.

    Now, have them all do an about face, so the new
    number reads (from the opposite side of the room)
    “21498367”

    Notice this number is also evenly divisible by 11.

    Now, put the kids in ANY order (while still
    maintaining boy-girl-boy-girl order).

    Is *THIS* new number even divisible by 11?

    Ask them why this *always* works? 😉

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