Proportion
An equation stating that two ratios are equal.
A proportion is a statement that two ratios are equal, such as a/b = c/d. The defining property is cross-multiplication: a × d = b × c, which lets you solve for any one unknown term. Proportions are the engine behind scaling recipes, converting units, reading maps, and any “if this, then how much of that” problem.
Example: If 2 pens cost $3, then x pens cost $9 → 2/3 = x/9 → x = 6 pens.