Ratios are figured with addition and subtraction and represent a comparison between numbers or a group of numbers. They are often represented as fractions and can be simplified as such using division.
There are three ways ratios can be expressed: x:y, x/y, or x to y. They all mean the same thing: for x, there is y. An example of this could be: there are 6 boys and 7 girls in math class. The ratio is 6:7, 6/7 or 6 to 7.
With ratios, the first figure is always the antecedent and the second is the consequent. Sometimes, ratios have to do with how the antecedent or consequent relate to the entire amount of the grouping. Using the example above, 6 boys + 7 girls = 13 kids in math class, so it could be said that 6/13 kids in math class are boys and 7/13 kids in math class are girls.
When added together, the two fractions from the classroom example equal 1. They equal 1 because 1 represents the whole grouping comprised of the antecedent and the consequent. When all six boys and seven girls are together in the classroom, it equals the whole of the math class.