A proportion is an equality between two ratios.
Constant of Proportionality
Properties of Proportions
In a proportion, the product of the means equals the product of the extremes (cross multiplication).
In a proportion of two or more equal ratios, the sum of the antecedents divided by the sum of the consequents is equal to any one of the ratios.
If the means and extremes in a proportion swap positions, the proportion does not vary.