She's trying to estimate the population mean, the mean age. She can't talk to all 700, so she takes a sample, a simple random sample of 20. So the n is equal to 20 here. From this 20, she calculates a sample mean of 38.75. Now ideally, she wants to construct a t-interval, a confidence interval using the t-statistic. And so that interval would look something like this. It would be the sample mean, plus or minus the critical value, times the sample standard deviation divided by the square root of n. And we use a t-statistic like this, and a t-table, and a t-distribution when we are trying to create confidence intervals for means where we don't have access to the standard deviation of the sampling distribution, but we can compute the sample standard deviation.