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An electrical firm manufactures light bulbs that have a length of life that is approximately normally...

  1. An electrical firm manufactures light bulbs that have a length of life that is approximately normally distributed with a standard deviation of 40 hours. If a sample of 30 bulbs has an average life of 780 hours,
  2. find a 96% confidence interval for the population mean of all bulbs produced by this firm.
  3. How large a sample is needed if we wish to be 96% confident that our sample mean will be within 10 hours of the true mean?
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