# What is the lower/upper limit of 90% confidence interval?

The lifetime of a certain brand of battery is known to have a standard deviation of hours. Suppose that a random sample of such batteries has a mean lifetime of hours. Based on this sample, find a confidence interval for the true mean lifetime of all batteries of this brand. Then complete the table below.

Carry your intermediate computations to at least three decimal places. Round your answers to one decimal place.

What is the lower limit of the 90% confidence interval?

What is the upper limit of the 90% confidence interval?

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#### Solution Preview

1. To calculate that the range, you need to use the appropriate formula, which in this case is your z formula (since you have a sample mean and a standard deviation for that sample). Your z formula is: z=(xbar-mu)/sigma,

where xbar is the mean of the sample you have, sigma is the standard deviation, and mu is what you're looking for - the true population mean. Normally, you would solve this equation for z to determine the likelihood of a certain sample mean, but in this case, ...

#### Solution Summary

In four detailed steps, this solution provides students with an explanation on how to discover the lower and upper limit of a 90% confidence interval, when considering a certain battery brand with a standard deviation of hours.