The manufacturer of a certain type of automobile claims that under typical urban driving conditions the automobile will travel, on the average, at least 20 miles per gallon of gasoline. The owner of an automobile of this type notes the mileages that he has obtained in his own urban driving when he fills his automobile's tank with gasoline on 9 different occasions. He finds that the results. in miles per gallon, for the different thank full were as follows: 15.6, 18.6, 18.3, 20.1, 21.5, 18.4, 19.1, 20.4, and 19.0. Test the manufacturer's claim by carrying out a test at the level of significance "0 = 0.05. List carefully the assumptions you must make.