Results 1 to 2 of 2

Math Help - Expected value

  1. #1
    Newbie
    Joined
    Jul 2009
    Posts
    14

    Expected value

    Assume there are 700 colors. 322 reds. 336 blues. 42 neutrals. We sample 70 of them.

    a) What is the probability of only getting only blues and reds?
    b) What is the expected value of the number of simple random samples of size 70 that must be drawn until (and including) the first sample that contains at least one color that is neither a blue nor a red if you repeatedly drawing simple random samples of size 70 (taking each simple random sample from the full population of 700)?

    Answers I got:
    a) .01038
    b) 1.01048887451749

    Can somebody explain to me how it's done?
    Last edited by Starry; July 26th 2009 at 09:55 PM.
    Follow Math Help Forum on Facebook and Google+

  2. #2
    Super Member

    Joined
    May 2006
    From
    Lexington, MA (USA)
    Posts
    11,905
    Thanks
    765
    Hello, Starry!

    Where did this problem come from?

    Part (a) has an answer that no one wants to crank out.


    There are 700 marbles: 322 reds. 336 blues. 42 neutrals. We sample 70 of them.

    a) What is the probability of only getting only blues and reds?

    There are 700 marbles; we sample 70 of them.
    . . There are: . {700\choose70} possible samples.

    There are 658 marbles that are red or blue; we want 70 of them.
    . . There are: . {658\choose70} samples that have only blues and/or reds.


    The probability is: . \frac{{658\choose70}} {{700\choose70}} \;=\;\frac{658!}{70!\,588!}\,\frac{70!\,630!}{700!  } \;=\;\frac{630!}{588!}\cdot \frac{658!}{700!}

    . . And the answer is: . \frac{630\cdot629\cdot629\cdots589}{700\cdot699\cd  ot698\cdots 659}


    I'll wait in the car . . .
    .
    Follow Math Help Forum on Facebook and Google+

Similar Math Help Forum Discussions

  1. Expected Value
    Posted in the Statistics Forum
    Replies: 1
    Last Post: April 25th 2011, 04:51 PM
  2. Expected value/ st dev/var
    Posted in the Advanced Statistics Forum
    Replies: 3
    Last Post: September 30th 2010, 03:55 AM
  3. Expected Value
    Posted in the Advanced Statistics Forum
    Replies: 9
    Last Post: September 20th 2010, 12:52 AM
  4. Expected Value
    Posted in the Statistics Forum
    Replies: 3
    Last Post: April 28th 2009, 05:05 PM
  5. expected value
    Posted in the Advanced Statistics Forum
    Replies: 1
    Last Post: January 16th 2009, 09:51 AM

Search Tags


/mathhelpforum @mathhelpforum