What Is a Proportion?
Definition
Proportion: An equation stating that two ratios are equal: a / b = c / x. When three terms are known, the fourth is found by cross multiplication.
A proportion is an equation stating that two ratios are equal: a / b = c / x, or a : b = c : x. When three terms are known, you find the fourth by cross multiplication. A proportion can be direct or inverse. For example, if 3 pens cost 12, then 5 pens cost 20.
Where it appears in the formulas
In a / b = c / x, cross multiplication gives a × x = b × c, so x = b × c / a. For 3 pens that cost 12, the proportion calculator finds x = 12 × 5 / 3 = 20 for 5 pens. In an inverse proportion the equation becomes a × b = c × x.
Direct and inverse proportion
In a direct proportion both quantities grow together and the ratio of matching values stays constant. In an inverse proportion one grows as the other shrinks, and the product of matching values stays constant.