Answer: The ratio of 6 /7 represent the number of boys to the number of girls in one classroom. You are given a total number of boys and girls.
So, your ratio has to represent how many boys to the total number of boys and girls in one classroom, or 13.
The proportion becomes, 6 / 13 = x / 78.
Cross multiply getting, 13 * x = 6 * 78
Divide by 13 to solve for x, or x = 36.
There are 36 boys in all of the classrooms and 42 girls in all of the classrooms.
The ratio of boys to girls in all of the classrooms is 36 / 42, or 6 / 7.