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A new extended-life light bulb has an average service life of 750 hours, with a standard deviation of 50 hours. If the service life of these light bulbs approximates a normal distribution, about what percent of the distribution will be between 600 hours and 900 hours?

  • statistics -

    Use z-scores.

    Formula:

    z = (x - mean)/sd

    Find two z-scores, using 600 for x and also 900 for x. Mean = 750 and sd = 50.

    Once you find the two z-scores, use a z-distribution table to determine your probability between the two z-scores.

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