What Is Proportion?
A proportion is an equation showing that two ratios are equal, usually written as a/b = c/d. Proportions are solved by cross-multiplying: a×d = b×c. They're the basis for scaling recipes, maps, and unit conversions, whenever one known ratio needs to be applied to a new quantity.
Example
In the proportion 2/3 = x/12, cross-multiplying gives 2×12 = 3×x, so 24 = 3x, and x = 8.