They calculate the sample mean, which is 700. They also calculate the sample standard deviation, which is equal to 50. And they wanna use this data to construct a 95% confidence interval. And so our confidence interval is going to take the form, and we've seen this before, our sample mean plus or minus our critical value times the sample standard deviation divided by the square root of n. The reason why we're using a t-statistic is because we don't know the actual standard deviation for the population. If we knew the standard deviation for the population, we would use that instead of our sample standard deviation. And if we use that, if we used sigma, which is a population parameter, then we could use a z-statistic right over here. We would use a z-distribution.