Results 1 to 8 of 8

Math Help - normal distribution: > and <

  1. #1
    Newbie
    Joined
    Jun 2009
    Posts
    18

    normal distribution: > and <

    How do you find P(X>111) and P(X<85) if the mean is 100 and the SD is 10? For the 1st one I did (111-100)/10=1.1 which is 0.1357 on the normal distribution table. For the 2nd one, I'm not so sure how to go about it. First I did (85-100)/10=-1.5. Since it is < rather than >, I just took the value of 1.5 from the normal distribution chart, 0.0668. Any suggestions? Thank you
    Follow Math Help Forum on Facebook and Google+

  2. #2
    MHF Contributor matheagle's Avatar
    Joined
    Feb 2009
    Posts
    2,763
    Thanks
    5
    Quote Originally Posted by madgab View Post
    How do you find P(X>111) and P(X<85) if the mean is 100 and the SD is 10? For the 1st one I did (111-100)/10=1.1 which is 0.1357 on the normal distribution table. For the 2nd one, I'm not so sure how to go about it. First I did (85-100)/10=-1.5. Since it is < rather than >, I just took the value of 1.5 from the normal distribution chart, 0.0668. Any suggestions? Thank you

    P(Z<-1.5)=P(Z>1.5) by symmetry.
    This is a very nice stat calculator...
    http://www.danielsoper.com/statcalc/calc20.aspx
    Plug in 1.5 and you get P(Z<-1.5)=P(Z>1.5)=.0668072
    Follow Math Help Forum on Facebook and Google+

  3. #3
    Newbie
    Joined
    Jun 2009
    Posts
    18
    great calculator! those were the answers i had actually handed in and they were incorrect.... so i figured i went wrong somewhere.
    Follow Math Help Forum on Facebook and Google+

  4. #4
    MHF Contributor matheagle's Avatar
    Joined
    Feb 2009
    Posts
    2,763
    Thanks
    5
    I thought your answers were correct.
    NOW, I'm confused.
    Follow Math Help Forum on Facebook and Google+

  5. #5
    Flow Master
    mr fantastic's Avatar
    Joined
    Dec 2007
    From
    Zeitgeist
    Posts
    16,948
    Thanks
    5
    Quote Originally Posted by matheagle View Post
    I thought your answers were correct.
    NOW, I'm confused.
    The correct (to seven decimal places) answer to P(X < 85) is indeed 0.0668072.

    So either:

    1. Whoever graded the work has made a mistake (possibly a TA working off a wrong answer sheet)

    2. The answer is 'too correct' ....
    Follow Math Help Forum on Facebook and Google+

  6. #6
    MHF Contributor matheagle's Avatar
    Joined
    Feb 2009
    Posts
    2,763
    Thanks
    5
    and I'm too tired, gn.
    Follow Math Help Forum on Facebook and Google+

  7. #7
    Grand Panjandrum
    Joined
    Nov 2005
    From
    someplace
    Posts
    14,972
    Thanks
    4
    Quote Originally Posted by madgab View Post
    great calculator! those were the answers i had actually handed in and they were incorrect.... so i figured i went wrong somewhere.
    Quote what you are responding to, that way your post just might make sense. As it is you leave us guessing what you handed in and had marked as wrong (use the 'quote' button below and to the right of the post you are looking at).

    CB
    Follow Math Help Forum on Facebook and Google+

  8. #8
    Grand Panjandrum
    Joined
    Nov 2005
    From
    someplace
    Posts
    14,972
    Thanks
    4
    Quote Originally Posted by mr fantastic View Post
    The correct (to seven decimal places) answer to P(X < 85) is indeed 0.0668072.

    So either:

    1. Whoever graded the work has made a mistake (possibly a TA working off a wrong answer sheet)

    2. The answer is 'too correct' ....
    You are making an assumption about which post madgab was responding to which may not be valid.

    CB
    Follow Math Help Forum on Facebook and Google+

Similar Math Help Forum Discussions

  1. Replies: 1
    Last Post: December 27th 2011, 02:08 PM
  2. normal distribution prior and posterior distribution proof
    Posted in the Advanced Statistics Forum
    Replies: 0
    Last Post: March 9th 2011, 07:12 PM
  3. Replies: 2
    Last Post: March 29th 2010, 03:05 PM
  4. Replies: 3
    Last Post: February 6th 2009, 04:59 AM
  5. Replies: 1
    Last Post: April 20th 2008, 07:35 PM

Search Tags


/mathhelpforum @mathhelpforum