Explanation of this formula: There are *** 18 girls. If the number of girls in the big class and the middle class doubles, the number of boys in the middle class will be the same. If you subtract the total number of boys from the total number of girls to be expanded in 18x2-32, there will be more girls than boys in the big class, because the ratio of boys to girls in the big class is 5:3, and the number of girls will double.