After a treatment of a certain species of rodents, a sample of 10 animals is selected and weighed. The

weights in grams are the following

83 , 81 , 84 , 80 , 85 , 87 , 89 , 84 , 82 , 80 .

It is known that untreated rodents have an average weight of 87.6 g. The weight of a rodent is modelled by a

normal random variable. At 5% significance level, test the hypothesis that "the treatment has no effect on

the mean weight" against "the treatment decreases the mean weight