I am reading the book Statistics by Witte & Witte. Some questions puzzle me.
The normal range for a widely accepted measure of body size, the body mass index (BMI), ranges from 18.5 to 25. Using the midrange BMI score of 21.75 as the null hypothesized value for the population mean, test this hypothesis at the .01 level of significance given a random sample of 30 weight-watcher participants who show a mean BMI = 22.2 and a standard deviation of 3.1.
The chapter explains the z test. It uses the following formula.
z = (mean_sam - mean_pop) / (stdev_pop / sqrt(n))
In the question only a standard deviation of the sample is given. I don't no whether and how this affects this formula.