Math

posted by Gigi

Q. When your doing a multiplication problem, for example 84 times 37 = 3,108, how do you check the answer without a calculator?
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A. I'm kinda confused on how to check the answer, do i do 84 times 400??

  1. Reiny

    here is a very old trick:
    add up the digits of each factor
    87---> 8+4 = 12 , do it again --- 1+2 = 3
    37 ----> 3+7 = 10 ---> 1+0=1
    multiply them : 3*1 = 3

    take the answer and add up the digits
    3108 ---> 3+1+0+8 = 12 -- 1+2 = 3

    same result!, extremely high probability that our answer is correct

    This is called "casting out nines"

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