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Math Help - Probability Question causing me a bit of bother

  1. #1
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    Probability Question causing me a bit of bother

    There's probably some really easy way to do this question but I spent about an hour at study this evening trying to do this question but I couldn't get ny head around it.... and yes it is annoying me at this point lol


    basically it reads...

    there are 11 black socks in a drawer and n white socks... if the probability of picking 2 n socks is 1/12 what is the value for n.

    Can anybody please help

    Thanking you
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  2. #2
    MHF Contributor
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    The probability of choosing 2 white socks from the drawer is:

    \frac{_nC_2}{_{11 + n}C_2} = \frac{1}{12}

    \frac{\frac{n(n-1)}{2}}{\frac{(11 + n)(10 + n)}{2}} = \frac{1}{12}

    \frac{n(n-1)}{(11 + n)(10 + n)} = \frac{1}{12}

    12n(n-1) = (11 + n)(10 + n)

    12n^2 - 12n = 110 + 21n + n^2

    11n^2 - 33n - 110 = 0

    11(n - 5)(n + 2) = 0

    n = 5
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  3. #3
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    thaning you


    could you possibibly help explain line 2 to me..


    EDIT
    i understand it now i got it out now

    thanking you

    thread can be locked now
    Last edited by johnmcdnl; March 25th 2010 at 11:49 AM.
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