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Math Help - Normal Distribution Probability.

  1. #1
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    Normal Distribution Probability.

    A study of long-distance phone calls made from the corporate offices of Pepsi Bottling Group, Inc., in Somers, New York, revealed the length of the calls, in minutes, follows the normal probability distribution. The mean length of time per call was 4.2 minutes and the standard deviation was 0.60 minutes.

    a) What fraction of the calls last more than 5 minutes? (Round z-score computation to 2 decimal places and your final answer to 4 decimal places.

    b) What fraction of the calls last between 5 and 6 minutes? (Round your answer to 4 decimal places.)

    c) What fraction of the calls last between 4 and 6 minutes? (Round z-score computation to 2 decimal places and your final answer to 4 decimal places.)

    d) As part of her report to the president, the director of communications would like to report the length of the longest (in duration) 4 percent of the calls. What is this time? (Round z-score computation to 2 decimal places and your final answer to 4 decimal places)

    I don't understand, I did one just like this with no problem...I feel like I'm doing exactly what the book says...gah! Someone help!
    Last edited by mr fantastic; October 25th 2010 at 06:27 PM. Reason: Edited title.
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    Quote Originally Posted by bubbletea View Post
    A study of long-distance phone calls made from the corporate offices of Pepsi Bottling Group, Inc., in Somers, New York, revealed the length of the calls, in minutes, follows the normal probability distribution. The mean length of time per call was 4.2 minutes and the standard deviation was 0.60 minutes.

    a) What fraction of the calls last more than 5 minutes? (Round z-score computation to 2 decimal places and your final answer to 4 decimal places.

    b) What fraction of the calls last between 5 and 6 minutes? (Round your answer to 4 decimal places.)

    c) What fraction of the calls last between 4 and 6 minutes? (Round z-score computation to 2 decimal places and your final answer to 4 decimal places.)

    d) As part of her report to the president, the director of communications would like to report the length of the longest (in duration) 4 percent of the calls. What is this time? (Round z-score computation to 2 decimal places and your final answer to 4 decimal places)

    I don't understand, I did one just like this with no problem...I feel like I'm doing exactly what the book says...gah! Someone help!
    Please show your work and answers.
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  3. #3
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    clue:Let X be
    the length of the calls, in minutes, then with X~N(4.2,0.60)

    Caluclate:
    1. P(X>5)=1-P(X<= 5)
    2. P(5<X<6)
    3 P(4<X<6)
    4. P(X>x)=0.04
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