The masses, in grams, of 13 ball bearings taken at random from a batch are: 21.4, 23.1, 25.9, 24.7, 23.4, 24.5, 25.0, 22.5, 26.9, 26.4, 25.8, 23.2, and 21.9. Calculate a 95% confidence interval for the mean mass of the population, supposed normal, from which these masses were drawn.