Results 1 to 4 of 4

Math Help - Normal distribution question

  1. #1
    Junior Member
    Joined
    Feb 2009
    Posts
    57

    Normal distribution question

    Scores on an examination are assumed to be normally distributed with a mean 78 and variance 36. If it is known that a students score exceeds 72, what is the probability that her score exceeds 84?

    Let Y denote the score, then P(Y>84 | Y>72) = P(z > .16 | z > -.16)
    I am stuck here and not sure what to do.
    Follow Math Help Forum on Facebook and Google+

  2. #2
    Rhymes with Orange Chris L T521's Avatar
    Joined
    May 2008
    From
    Chicago, IL
    Posts
    2,844
    Thanks
    3
    Quote Originally Posted by vexiked View Post
    Scores on an examination are assumed to be normally distributed with a mean 78 and variance 36. If it is known that a students score exceeds 72, what is the probability that her score exceeds 84?

    Let Y denote the score, then P(Y>84 | Y>72) = P(z > .16 | z > -.16)
    I am stuck here and not sure what to do.
    This is conditional probability, so we see that

    \mathbb{P}\!\left(z>.16|z>-.16\right)=\frac{\mathbb{P}\!\left((z>-.16)\cap (z>.16)\right)}{\mathbb{P}\!\left(z>-.16\right)}=\frac{\mathbb{P}\!\left(z>.16\right)}{  \mathbb{P}\!\left(z>-.16\right)}

    Can you take it from here?
    Follow Math Help Forum on Facebook and Google+

  3. #3
    Junior Member
    Joined
    Feb 2009
    Posts
    57
    So looking up .16 in the table gives .4364/.4364 giving us a value of 1. This is where I am confused.
    Follow Math Help Forum on Facebook and Google+

  4. #4
    Flow Master
    mr fantastic's Avatar
    Joined
    Dec 2007
    From
    Zeitgeist
    Posts
    16,948
    Thanks
    5
    Quote Originally Posted by vexiked View Post
    So looking up .16 in the table gives .4364/.4364 giving us a value of 1. This is where I am confused.
    The denominator is NOT Pr(Z > 0.16), it's Pr(Z > -0.16).
    Follow Math Help Forum on Facebook and Google+

Similar Math Help Forum Discussions

  1. Normal Distribution question
    Posted in the Statistics Forum
    Replies: 5
    Last Post: August 31st 2011, 07:49 PM
  2. Another question on normal distribution
    Posted in the Statistics Forum
    Replies: 12
    Last Post: September 15th 2010, 03:26 AM
  3. Question on normal distribution
    Posted in the Statistics Forum
    Replies: 4
    Last Post: September 15th 2010, 12:39 AM
  4. Normal Distribution question
    Posted in the Advanced Statistics Forum
    Replies: 2
    Last Post: April 16th 2010, 06:11 AM
  5. Normal Distribution question
    Posted in the Statistics Forum
    Replies: 5
    Last Post: October 16th 2009, 09:29 PM

Search Tags


/mathhelpforum @mathhelpforum