How does a "for every" statement show a ratio comparison?

Example: For every 3 boys, there are 4 girls.

Ratio: 3:4 also 3/4

How to use it?

Example question: If there are 3 boys for every 4 girls and there are 27 boys in the class, how many girls are there?

How to set up the ratio:

3 27
— = —
4 x

Solve it?

[Cross Multiply]

3x= 108

[Divide]

X= 36

How to check it?

Reduce 27/36 and see if it equals 3/4

OR

Plug both into the calculator and both should come out with the same decimal. In this case it is 0.75

12.5 should be the anwser