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Math Help - Normal distribution

  1. #1
    Mar 2010

    Normal distribution

    I've done these type of questions before, but haven't touched it for a year and I forgot about it.. so can someone recall my memories ?

    Many residents of suburban neighbourhoods own more than one car but consider one of their cars to be the
    main family vehicle. The age of these family vehicles can be modelled by a Normal distribution with mean
    2 years and standard deviation 6 months.

    a) Approximately what percentage of family vehicles is between 1 and 3 years old?

    b) What is the standardised value for a family vehicle that is 3 years and 3 months old?

    Show working for me, so I understand how it's done! Argh, so frustrating
    Last edited by mr fantastic; April 15th 2010 at 04:01 PM. Reason: Changed post title
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  2. #2
    Senior Member
    Nov 2009
    If the car is 1 year old, it is 2 standard deviations away from the mean (2 x 6 months = 1 year, mean = 2 years, 2 - (2 * 6 months = 1 year) = 1 year.

    The same goes for 3 years old, just with the deviation on the other side.
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  3. #3
    Junior Member
    Sep 2009
    Think back to the formula for a standardized test statistic.
    test statistic = (statistic - param)/standard deviation

    So if you are looking for z, and you have the mean of the sample x-bar, the mean of the population mu, and the standard deviation, it should be as simple as plugging these values into the formula.
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  4. #4
    Mar 2010
    Sorry people, I'm really lost here.. can you please explain in more details?

    By the way.. what's param?
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