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Math Help - Normal distributions

  1. #1
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    Normal distributions

    Hi, I have several problems I would like some help.

    PROBLEM 1: Extruded plastic rods are automatically cut into lenghts of 6 inches. Actual lengths are normally distributed about a mean of 6 inches and their standard deviation is 0.06 inch.

    1- what proportion of the rods have lenghts that are outside the tolerance limits of 5.9 and 6.1 inches
    ?

    Here I did:
    p=F((6.1-6)/0.06)- F((5.9-6)/0.06)= F(1.67)-F(-1.67)=0.9525-0.0475=0.905

    P(outside tolerance)=1-0.905=0.095

    2- To what value does the standard deviation needs to be reduce if 99% of the rods must be within the tolerance?
    I can not fin this question.

    PROBLEM 2:
    In a photographic process, the developping time of prints may be looked upon as a random variable having the normal distribution with a mean of 16.28 seconds and a standard deviation of 0.12 second.
    - For which value is the probability 0.95 that it will be exceeded by the time it takes to develop one of the prints?

    I dont get this one.

    Can I have some help please?
    Last edited by braddy; April 2nd 2007 at 06:50 AM.
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