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Math Help - multiplying 2 numbers

  1. #1
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    multiplying 2 numbers

    523 * 489 using mental math

    523 * 500 = 52300 + 52300 + 52300 + 52300 =

    11 less groups... sounds complicated. Any other ideas?

    Thanks,

    Terminator
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  2. #2
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    Re: multiplying 2 numbers

    500 is half of one thousand so 500 times 523 is 1/2 of 523000. Can you do 523000/2 in you head?

    489 is 11 less than 500 so after you have 500 times 523, you still need to subtract 11 times 523. What is 10 times 523? Can you subtract that from the 523000/2 you already have? The subtract it once more.
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  3. #3
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    Re: multiplying 2 numbers

    anything easier? I cannot do 523000 in my head...
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  4. #4
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    Re: multiplying 2 numbers

    Hello, terminator!

    523 \times 489\;\text{ using mental math}

    I might be able to do this mentally ... under threat of death.


    (523)(489) \;=\;(500+23)(500-11)

    . . . . . . . . . . . =\;500^2 - (11)(500) + (23)(500) - (23)(11)

    . . . . . . . . . . . =\;500^2 + (12)(500) - (23)(11)

    . . . . . . . . . . . =\;250,\!000 + 6,\!000 -253

    . . . . . . . . . . . =\;256,\!000 - 253

    . . . . . . . . . . . =\;255,\!747

    Last edited by Soroban; February 4th 2013 at 03:23 PM.
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  5. #5
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    Re: multiplying 2 numbers

    firstly;
    you can do this

    532
    x 489
    ----
    4788
    4256
    2128
    ----------
    260148
    -----------
    9x2=18
    9x3=27
    9x5=45

    8x2=16
    8x3=24
    8x5=40

    4x2=8
    4x3=12
    4x5=20

    answer is = 260148

    i believe this method is very easy and you just have to pick one number from the bottom and multiply it by all the numbers above
    the do the same for the next bottoms number and after all that, you add all of the answer

    let me know if you do not understand!!

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  6. #6
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    Re: multiplying 2 numbers

    Quote Originally Posted by terminator View Post
    anything easier? I cannot do 523000 in my head...
    Seriously? 5 divided by 2 is 2 with remainder 1. 12 divided by 2 is 6 with remander 0. 3 divided by 2 is 1 with remainder 1 and 1000 divided by 2 is 500:
    523000 divided by 2 is 261500.
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