A ratio is just making a comparison between different quantities.
example:
Say there are 10 students in a class. 4 students are girls, 6 are boys.
the ratio of girls to boys is 4:6
then you simplify to 2:3
the ratio of boys to girls is flipped because boys is said first.
boys to girls 6:4 simplified to 3:2
If you are asked for the ratio of girls to students then it's 4:10 because you use the number of all students in the class.
4:10 simplifies to 2:5
hope this helps