In a certain school the ratio of girls to boys is 5:4. This ratio is the same in every grade.
In the grade three class there are 30 girls. Since the 5 6 = 30 and the ratio of girls to boys is 5:4 there must be 4 6 = 24 boys in grade 3. Thus 5:4 and 30:24 are equal ratios. (I would actually say equivalent ratios.)
If there are 25 girls in grade 4 how many boys are there? In other words what is ?? is 5:4 and 25:?? are equivalent ratios?