# Homework 12 Question 3

Justin is interested in buying a digital phone. He visited 17 stores at random and recorded the price of the particular phone he wants. The sample of prices had a mean of 174.54 and a standard deviation of 27.05.

I'm struggling with part B

Calculate a 95% confidence interval for the mean price of this model of digital phone:

(Enter the smaller value in the left answer box.)

I was using the formula (measured-assumed)/(27.05/sqrt(17))

How do I calculate the confidence interval with only one mean?

Thanks!

## Comments

I added some parentheses, which are important for that particular formula. Having said that, I think the formula that you want is

$$\overline{x} \pm t^* \times s/\sqrt{n},$$

where $\bar{x}$ is the sample mean, $s$ is the sample standard deviation, and $n$ is the sample size - all stated in the problem. I guess the last piece of the puzzle the $t^*$-multiplier, which you can read off of our $t$-table.