This document defines and explains ratio, proportion, and the different types of variation. It contains the following key points: 1. A ratio compares two quantities using division, where the quantities must be in the same unit. 2. A proportion states that the ratios of two sets of quantities are equal. The product of the extremes equals the product of the means. 3. Direct variation occurs when quantities change in the same direction. Inverse variation occurs when they change in opposite directions. 4. A constant of proportionality describes the mathematical relationship between variables in direct or inverse variation.