The lifetimes in hours of two brands of batteries follow normal distributions with means x, y respectively and equal but unknown variances. Independent random samples of 5 batteries from each brand are taken and the following data are observed. Construct a 90% confidence interval for the difference X - μy. Brand X Y (-7.94,47.94) (-13.27,53.27) (-16.66,56.66) (-17.20,57.20) n 5 5 Sample mean 210 190 Sample variance 840 760

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