5The lifetime of a certain brand ofbatteries is known to be normally distributed. A sample of21 batteries gives a mean lifetime of 116.0 hours with a standard deviation of9.0 hours. Use Excel to calculate thelowerlimit of a 95% confidence interval for the mean lifetime of batteries of this brand.

Give your answer to1 decimal place.

We are standardisingx

withS

n

so we will need to use the t distribution to calculate t* (rather than using the Standard Normal distribution to calculate z*).