Ratios

Ratios are comparing two things together.  Let's take comparing boys to girls in a classroom.  There are 27 students in class. Of the 27 students, 15 are boys and 12 are girls.  These ratios may be represented as:

15 boys to 12 girls
For every 15 boys, there are 12 girls (however, only used when comparing larger quantities through scaling)

15 boys : 12 girls

15 boys
12 girls

These ratios all compare part to part.

It is important to keep the same order in which was given in the problem.  Notice I said:

"Let's take comparing boys to girls in a classroom".

I kept the same order because that was what was given to me in the problem.

You can compare ratios in terms of part to whole as well.

15 boys out of 27 students
12 girls out of 27 students

In this case, we used part of the classroom (boys or girls) and compared it to the total amount of students.  Since we know that there are only 2 genders, if I said 15 boys out of 27 students we could subtract the part FROM THE whole to find the amount of the other part.

 27 total students
-15 boys
12 girls.

These are the basics to ratios.