A mathematics test, the average score of the class is 91.2 points, known 21 girls, the average score of 92 points per person, boys

A mathematics test, the average score of the class is 91.2 points, known 21 girls, the average score of 92 points per person, boys

Original question: a math test, the average score of the class is 91.2 points, known that there are 21 girls, 92 points per person, 90.5 points per man, ask how many boys there are in this class
Equation: let a be male
（a+21）×91.2=a×90.5+92×21
91.2a+91.2×21=90.5a+92×21
0.7a=0.8×21
A = 24
Arithmetic: girls higher than the average score, a total of (92-91.2) × 21 = 16.8 points
The average score of boys is 91.2-90.5 = 0.7
So there are 16.8 / 0.7 boys = 24