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What is the approximate percentile rank of a lightbulb that failed at 9000 hours if the number of lightbulbs is 400? The number of lightbulbs that lasted between 5400 and 9000 were about 326 lightbulbs. The mean is 7800 and the standard deviation is 1200

My guess was: percentile = 9000 hours/400 lightbulbs = 22.5, but I'm not sure if I'm solving this right

  • Math -

    This is from the actual problem - A consumer product testing lab tested 400 13-watt compact
    fluorescent light bulbs by turning them all on at once, leaving them
    on until they burned out, and recording the time-to-failure of each
    bulb. The test revealed a symmetric and mound-shaped
    distribution of times-to-failure with mean 7800 hours and standard
    deviation 1200 hours. The number of lightbulbs that lasted between 5400 and 9000 hours were about 326 lightbulbs.

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