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Thread: Combinatorics - Counting

  1. #1
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    Question Combinatorics - Counting

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    I was browsing the web to find intriguing questions for quant section especially in Combinatorics.
    Bumped on to the following question.

    How many ternary strings of length 4 have exactly one 1?
    [Courtesy : http://web.eecs.utk.edu/~booth/311-0...natorics.html]

    My answer for this question was,
    _,_,_,_ 4 spaces needs to be filled with 0,1 and 2
    out of which one of them needs to be 1. there are 4 ways in which one of them could be 1
    and in all other instances there would be choices of 2 numbers in filling 3 of these blanks
    4*(23)


    32 strings have exactly one 1 in it.

    According to the site where i got this question from though, the answer is different.
    [3*(23)]

    Can anyone clarify what's the logical fallacy in my answer? or elaborate more on why it could be wrong?

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    GRE-GMAT Instructor Brent Hanneson's Avatar
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    Re: Combinatorics - Counting

    Quote Originally Posted by pranavanmaru View Post

    How many ternary strings of length 4 have exactly one 1?
    Way out of scope for the GRE.

    Cheers,
    Brent - Greenlight Test Prep

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