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October 21, 2014

October 21, 2014

Posted by **alice** on Wednesday, December 5, 2012 at 1:36pm.

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

- Math -
**alice**, Wednesday, December 5, 2012 at 1:50pmThis 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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