11-28. A horticulturalist has been using brand A fertilizer on a certain variety of plant. A new brand B fertilizer has just been marketed, which purports to be superior to brand A. Two independent groups of n1 = n2 = 8 new plantings are to be treated (one with the brand A fertilizer and the other with the brand B fertilizer) and the times (in days) taken to achieve a certain height are recorded:

Brand A: 30 35 30 31 39 39 35 29 Brand B: 30 33 29 33 35 28 30 29 Find a 99% confidence interval for the true mean difference in growth time (given that these times are approximately normally distributed and the variances are approximately equal).