21. A school has six elementary school grades, with 8 classes in each grade; three junior high school grades, with 8 classes in each grade; and three senior high school grades, with 12 classes in each grade. Now, 27 classes are to be selected for a survey, ensuring that the proportion of classes selected from each type is the same. Therefore, from the elementary school, $\qquad$ classes should be selected from each grade, and from the junior high school, $\qquad$ classes should be selected from each grade.