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Math Help - Probability

  1. #1
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    Probability

    The manufacturer of a brand of light bulbs claims that the life of its bulbs is normally distributed with mean of 700 hours and standard deviation of 120 hours. What is the probability that the light bulb that you just replaced in your upstairs hallway will last at least 880 hours?

    i cant remember how to do this! anyone know how?
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  2. #2
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    Use z=\frac{x-{\mu}}{\sigma}

    z=\frac{880-700}{120}=1.5

    Now, look up z=1.5 in a table of z-scores.

    z=.9332.

    But, we want 'at least', so we subtract that from 1 and get .0668

    The probability of it lasting at least 880 hours is about 7%

    See now?.
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  3. #3
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    thanks!
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