Answer: 60 teachers
=========================================================
Step-by-step explanation:
There are 500 female students and the ratio of boys to girls is 2:5
Multiply both parts of that ratio by 100 so that the "5" turns into "500"
The ratio 2:5 turns into 200:500 showing that for every 200 male students there are 500 female students. There are 200+500 = 700 students total.
Because of this 700 figure, we'll need to multiply both parts of the ratio 3:35 by 20 so that the 35 becomes 700.
The ratio 3:35 turns into 60:700. For every 60 teachers, there are 700 students.
----------------------
Another way to solve:
The ratio of boys to girls is 2:5 which we can write as 2/5
x = number of boys
2/5 = x/500
2*500 = 5x
1000 = 5x
5x = 1000
x = 1000/5
x = 200
There are 200 boys and 500 girls, so 200+500 = 700 total.
y = number of teachers
3/35 = y/700
3*700 = 35y
2100 = 35y
35y = 2100
y = 2100/35
y = 60
There are 60 teachers