Suppose the height of college students can be modeled by the normal distribution. We believe the mean of the normal distribution is 1.76 meters. We randomly sample 18 students and find an average of 1.69 meters and a sample standard deviation of 12 cm (=.12 meters)

Find an 80% confidence interval for mu.

Why isn't mu given in this interval.

Would this be used for a t-dist or for a z-score?

Since there is a sample its a t-dist correct? But how do you compute the sigma for the population in order to find the CI?