What Is Ratio?
A ratio compares two (or more) quantities, showing their relative sizes, often written as a:b or a/b. Ratios can be simplified the same way fractions can, by dividing both sides by their greatest common factor, and they stay meaningful even when the actual quantities scale up or down together.
Example
A recipe using 2 cups of flour to 1 cup of sugar has a ratio of 2:1. Doubling the recipe to 4 cups of flour and 2 cups of sugar keeps the same 2:1 ratio.