Results 1 to 2 of 2

Math Help - Mean and Variance

  1. #1
    Newbie
    Joined
    Feb 2008
    Posts
    16

    Mean and Variance

    A secretary was given n passwords and tries the passwords at random. Exactly one password will permit access to a computer file. Find the mean and variance of Y, the number of trials required to open the file, if unsuccessful passwords are eliminated.
    What I have for this question is that the probability of getting the password right on:
    the 1st try=1/n
    2nd try=1/n
    3rd try=1/n
    so thus the mean is:
    =1(1/n)+2(1/n)+3(1/n)+y(1/n)
    = 1+2+3+4+y
    n
    is this right or can i go further?
    thanks
    Follow Math Help Forum on Facebook and Google+

  2. #2
    MHF Contributor

    Joined
    Aug 2006
    Posts
    18,605
    Thanks
    1574
    Awards
    1
    \sum\limits_{k = 1}^n {k\left( {\frac{1}<br />
{n}} \right)}  = \left( {\frac{1}<br />
{n}} \right)\sum\limits_{k = 1}^n k  = \left( {\frac{1}<br />
{n}} \right)\left( {\frac{{n\left( {n + 1} \right)}}<br />
{2}} \right) = \frac{{\left( {n + 1} \right)}}<br />
{2}

    \sum\limits_{k = 1}^n {k^2 \left( {\frac{1}<br />
{n}} \right)}  = \left( {\frac{1}<br />
{n}} \right)\sum\limits_{k = 1}^n {k^2 }  = \left( {\frac{1}<br />
{n}} \right)\left( {\frac{{n\left( {n + 1} \right)\left( {2n + 1} \right)}}<br />
{6}} \right)<br />

    Use these to find E & V.
    Follow Math Help Forum on Facebook and Google+

Similar Math Help Forum Discussions

  1. Help with variance
    Posted in the Advanced Statistics Forum
    Replies: 2
    Last Post: September 3rd 2011, 01:53 AM
  2. variance of the sample variance
    Posted in the Advanced Statistics Forum
    Replies: 3
    Last Post: February 8th 2011, 08:35 PM
  3. How to get new variance given old variance and average
    Posted in the Advanced Statistics Forum
    Replies: 0
    Last Post: November 23rd 2010, 05:39 AM
  4. Variance Help
    Posted in the Statistics Forum
    Replies: 2
    Last Post: August 23rd 2009, 01:22 PM
  5. Variance of the population variance.
    Posted in the Advanced Statistics Forum
    Replies: 2
    Last Post: November 10th 2008, 09:07 AM

Search Tags


/mathhelpforum @mathhelpforum