Confidence interval = mean ± tc{S / √n}
= 28,000 ± (1.98) (4,250 / √100) or 27,159 to 28,842
If you use a z-statistic because of the large sample size, you get 28,000 ± (1.96) (4,250 / √100) = 27,167 to 28,833, which is closest to the correct answer.
