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  • Colourful numbers

    Three-digit numbers

    1 May, 1998
    May 1998

    A three-digit number is such that its second digit is the sum of its first and third digits.

    Prove that the number must be divisible by 11.

    Solution

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    Anonymous

    13 June 2011

    Permalink
    Comment

    All right, but how can you prove that a number like 62345678987654555548 (or even a bigger number) is divisible bij 11?

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    Anonymous

    18 August 2011

    In reply to three-digit numbers by Anonymous

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    Comment

    yes.
    1 way: Use calculator for windows 7, which is totally capable of numbers larger than that.(12 digits longer than 62345678987654555548)
    the other speedy way:
    What is:
    6 - 2 + 3 - 4 ...?
    If it is 0 or 11 or -11, it is divisible...
    if it isn't, no.

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    Anonymous

    21 November 2013

    In reply to yes... by Anonymous

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    Comment

    It can be done with 121 1+1=2 and 121 is divisble by 11

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    Anonymous

    30 August 2014

    In reply to three-digit numbers by Anonymous

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    Comment

    You can, in this way:

    First (from the first digit), divide the no: into groups of 2.

    Eg: 62 34 56 78 98 76 54 55 55 48

    Then, add them up.

    Eg: 62 + 34 + 56 + 78 + 98 + 76 + 54 + 55 + 55 + 48 = 616

    If the sum is more than 100, repeat step 1 and 2.

    Eg: 6 16
    6 + 16 = 22

    If the sum obtained is divisible by 11, then the initial no: is divisible by 11.

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    Anonymous

    13 July 2011

    Permalink
    Comment

    Let digits are a,b,c
    Let b=a+c because second digit is sum of last and first
    So number=a*100+b*10+c*1
    =a*100+(a+c)*10+c*1
    =a*100+a*10+c*10+c*1
    =110*a+11*c
    =11*10*a+11*c
    =11*(10*a+c)
    That is the number is multiple of 11 so it is divisible by 11

    opksalu@gmail.com

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