a study has been made to compare the nicotine contents of two brands of cigarrets. fifty cigarretes of brand A had an average nicotine content of 3.1 milligrams with a standard deviation s=0.7 milligram, while sixty cigarettes of brand B had an average nicotine content of 2.7 milligrams with standard devation s=1.0 milligram. construct a 95% confidence interval for the difference between the mean nicotine content of two brands of cigarettes.