We considered the change in the number of days exceeding 90'F from 1948 and 2018 at 197 randomly sampled locations from the NOAA database in Exercise 7.19. The mean and standard deviation of the reported differences are 2.9 days and 17.2 days

(a) Calculate a 50% confidence interval for the average difference between number of days exceeding O'F between 1948 and 2018. We've already checked the conditions for you