The useful life of a certain type of light bulb is known to have a= 40 hours. Hoe large a sample should be taken standard deviation of it is desired to have a margin of error of 10 hours or less at a 95% level of confidence?

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The useful life of a certain type of light bulb is known to have a= 40
hours. Hoe large a sample should be taken standard deviation of it is
desired to have a margin of error of 10 hours or less at a 95% level
of confidence?

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