(Solved): The lifetime of a certain brand of battery is known to have a standard deviation of 19.8 hours. ...

The lifetime of a certain brand of battery is known to have a standard deviation of $$19.8$$ hours. Suppose that a random sample of 80 such batferies has a mean lifetime of $$42.6$$ hours. Based on this sample, find a $$99\%$$ confidence interval for the true mean lifetime of all batteries of this brand. Then give its lower limut and upper limit. Carry your intermediate computations to at least three decimal places. Round your answers to one decimal place. (If necessary, consult a list of for a )