Answer:
35:9
Explanation:
The ratios can be combined by making the common element be represented by the same number.
Adjusted ratios
The common element in the two ratios is the number of girls. In the first ratio, girls are represented by 3 ratio units. In the second ratio, they are represented by 7 ratio units. Multiplying the first ratio by 7 and the second ratio by 3 will make the number of girls the same in both.
boys : girls = 2 : 3 = 14 : 21
girls : teachers = 7 : 3 = 21 : 9
Then the extended ratio can be written ...
boys : girls : teachers = 14 : 21 : 9
The number of students is the total of the numbers of boys and girls, so we have ...
students : teachers = (14 +21) : 9
students : teachers = 35 : 9