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Math Help - Finding X Given Normal Probability

  1. #1
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    Finding X Given Normal Probability

    Hello everyone,

    I have a question on a normal distribution problem where I have to find X given a certain probability. I know how to do it, but it seems like the method I used to achieve the answer is incorrect, and I don't understand why. I hope you guys can point me to the right direction

    Finding X Given Normal Probability-photo.jpg

    I have posted an image with the problem above and my way of solving it vs. professor's way.


    What I'm confused about: Why is this a left hand tail problem instead of a right hand tail like my way of doing it? Does the problem give out a hint stating that it's a left hand tail?
    Attached Thumbnails Attached Thumbnails Finding X Given Normal Probability-photo.jpg  
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  2. #2
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    Re: Finding X Given Normal Probability

    98% last longer than X hours, therefore only 2% have a shorter life.
    The way you have done it is the opposite way around i.e.only 2% last longer than X hours, therefore 98% have a shorter life.
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  3. #3
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    Re: Finding X Given Normal Probability

    Quote Originally Posted by pickslides View Post
    98% last longer than X hours, therefore only 2% have a shorter life.
    The way you have done it is the opposite way around i.e.only 2% last longer than X hours, therefore 98% have a shorter life.
    How do i know if it's left hand tail or a right hand tail?
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