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Math Help - Probability (for socks)

  1. #1
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    Probability (for socks)

    Suppose there are 4 blue, 4 white and 4 gray socks in a dresser. For each of the following, determine an arithmetic expression (for example 1/2 * 1/2)

    a.) Pulling at sock at random, whats the probability that that sock is white?

    b.) Pulling a sock at random, whats the probability its not white?

    c.) Pulling a sock at random, whats the probability that it is gray or white?

    d.) Pulling a sock at random, whats the probability that its white and gray?

    e.) Pulling out 2 socks (at random) in a row, whats the probability both are gray?

    f.) Pulling out 2 socks (at random) in a row, whats the probability that 1 is blue and then the other is gray?

    g.) Pulling out 2 socks (at random) in a row, whats the probability that itdoes not have a matching pair?

    h.) Pulling out 2 socks (at random) in a row, whats the probability that you do have a matching pair?

    i.) Determine the number of socks you have to remove at random to guarantee that you would have a pair.


    What I think:

    a.) 1/3
    b.) 1 - 1/3 = 2/3
    c.) 2/3
    d.) 0 since you're only picking out 1? Maybe hte professor meant pulling 2 socks at random - if thats the case, what would it be
    e.) (1/3)(1/3) = 1/9
    f.) (1/3)(1/3) = 1/9
    g.) Not sure
    h.) not sure
    i.) not sure
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  2. #2
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    i think e and f is wrong should be. This is my answer :

    e. 1/3 x 3/11 = 3/33 = 1/11
    f. 1/3 x 4/11 = 4/33
    g. 1/3 x 8/11 = 8/33
    h. 1 - 8/33 = 25/33
    i. My answer is 4 socks
    No of pigeonhole = 3
    ceiling function( x/3 ) = 2
    Therefore x = 4
    Last edited by tester85; December 10th 2008 at 08:15 AM.
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