Results 1 to 2 of 2

Math Help - How to find the unknown deviation from this problem!

  1. #1
    Newbie
    Joined
    Sep 2012
    From
    UK
    Posts
    20

    How to find the unknown deviation from this problem!

    I am a bit stuck on how to find the standard deviation of this question. I know the formula u need to use: x-(x bar) / Z .. But I dont know how to find the Z score!

    Could you please help me with (1) find the Z score value and (2) getting to the answer...

    The question being:
    Monica walks to work from home and she uses route a or route b.
    Her journey time, Y minutes by route B may be assumed to be normally distributed with a mean of 40 and an unknown standard deviation.
    Given that P(Y > 45) = 0.12, calculate the standard deviation value.

    I would really appreciate some helpful feedback and solutions!

    Thanks in advance
    Dan
    Follow Math Help Forum on Facebook and Google+

  2. #2
    MHF Contributor
    Joined
    Sep 2012
    From
    Australia
    Posts
    4,159
    Thanks
    760

    Re: How to find the unknown deviation from this problem!

    Hey Danthemaths.

    P(Y > 45) = 0.12 implies
    P(Y - 40 > 45 - 40) = 0.12 implies
    P([Y - 40]/sigma > (45-40)/sigma) = 0.12 implies
    P(Z > 5/sigma) = 0.12 implies
    P(Z > z) = 0.12 where 5/sigma = z which finally implies
    P(Z < z) = 1 - P(Z > z) = 1 - 0.12 = 0.88

    Now use Normal tables to get the value of z and use the fact that z = 5/sigma or in other words, sigma = 5/z.

    This is a tricky question.
    Follow Math Help Forum on Facebook and Google+

Similar Math Help Forum Discussions

  1. Estimation (Small sample, standard deviation unknown)
    Posted in the Advanced Statistics Forum
    Replies: 0
    Last Post: February 8th 2010, 05:50 AM
  2. Normal distribution with an unknown mean and standard deviation
    Posted in the Advanced Statistics Forum
    Replies: 1
    Last Post: October 1st 2009, 03:13 AM
  3. Confidence interval-population standard deviation unknown
    Posted in the Advanced Statistics Forum
    Replies: 3
    Last Post: April 4th 2008, 08:25 PM
  4. Replies: 1
    Last Post: April 4th 2008, 11:39 AM
  5. Find unknown
    Posted in the Algebra Forum
    Replies: 2
    Last Post: November 3rd 2007, 04:03 AM

Search Tags


/mathhelpforum @mathhelpforum