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Thursday, 4 August 2011

Math GRE - #3

A drawer contains 2 blue, 4 red, and 2 yellow socks. If 2 socks are to be randomly selected from the drawer, what is the probability that they will be the same colour?

  • \frac{2}{7}
  • \frac{2}{5}
  • \frac{3}{7}
  • \frac{1}{2}
  • \frac{3}{5}

    The probability of getting 2 blue in a row is: \frac{2}{8}\frac{1}{7}=\frac{1}{28}.

    The probability of getting 2 yellow in a row is: \frac{2}{8}\frac{1}{7}=\frac{1}{28}.

    The probability of getting 2 red in a row is: \frac{4}{8}\frac{3}{7}=\frac{6}{28}.

    So the total probability is: \frac{1}{28}+\frac{1}{28}+\frac{6}{28}=\frac{2}{7}.

    6 comments:

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    [hide] sean said...

    Paul, I like this blog. Perhaps you could hide the answers to the questions below a link?

    on 4 August 2011 at 15:52
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    [hide] Paul Liu said...

    Yup, starting tomorrow, I'll hide the answers behind spoiler tabs.

    on 4 August 2011 at 19:20
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    [hide] Adam said...

    There's another way to do it.

    Using (n k) to stand for the binomial coefficient.

    The answer is

    (2 2) + (2 2) + (4 2)
    ---------------------
    (28 2)

    =

    (# yellow pairs) + (# blue pairs) + (# red pairs)
    -------------------------------------------------
    (# total pairs)

    on 4 August 2011 at 20:42
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    [hide] Adam said...

    Uh, that should be (8 2) in the denominator. Durr.

    on 4 August 2011 at 20:50
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    [hide] Anonymous said...

    Hey Paul, why we multiplied by 1/7 while getting the value for each of the socks.

    on 16 October 2012 at 19:25
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    [hide] Paul Liu said...

    We are choosing two socks each time, so we need to multiply the probability of choosing the first sock (something out of 8 with the prob. of choosing the second sock (something out of 7).

    on 16 October 2012 at 22:26
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