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    Amaze Your Friends! 3 Cool Multiplication Tricks You Can Do In Your Head
    By Garth Sundem | July 9th 2009 06:04 AM | 2 comments | Print | E-mail | Track Comments
    About Garth

    Garth Sundem is a Science, Math and general Geek Culture writer, TED speaker, and author of books including Brain Trust: 93 Top Scientists Dish the...

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    Even though you're already the mathematical Wizard of Oz, you can still benefit from the Wow factor of hoisting a new curtain of number tricks to impress your friends and intimidate your enemies.

    Here, dear geek, are three nifty mind widgets to make mates want you and peers want to be you.

    Multiply up to 20x20
    1. For example, take 17x13
    2. Place the larger number on top, in your head
    3. Imagine a box, encompassing the 17 and the 3
    4. Add these to make 20
    5. Add a zero to this, to make it 200
    6. Multiply the 7 and the 3 to get 21
    7. Add this to 200 to get the answer: 221

    Multiply any two-digit number by 11
    1. For example, take 79
    2. Add a space between the two digits, making 7___9
    3. Add 7 and 9 to get 16
    4. Put 16 into the space (remember to carry the tens digit!)
    5. The answer is 869

    Multiply by 9 on your fingers
    1. Lay your spread fingers on a table in front of you
    2. To multiply by 4, fold down the fourth finger from the left
    3. Now read your fingers: there are three to the left of the fold and six to the right—this makes 36.

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    Comments

    Prof.Smalls
    check out vedic maths for more cool tricks
    You can multiply any number by eleven in your head, quite easily.

    For example, we'll take a seven digit number, 1415926, and multiply it by eleven.
    Start with the rightmost digit, 6, that is the last digit.
    Add the 6 to the 2 preceding it, equalling eight.
    Add the two and the nine preceding it, equalling 11, keep the one.
    Add the nine and the five preceding it, equalling 14, keep the four, add the 1 left over from the previous operation.
    Add the five and the one preceding it, equalling 6, and add the one left over from the previous operation.
    Add the one and the four preceding it, equalling 5.
    Add the four and the one preceding it, equalling 5.
    And finally, put the first digit, 1, as the first digit of the new number, giving 15575186.