This document discusses modeling using variation, including direct variation, inverse variation, and joint variation. It provides examples of solving variation problems by writing the general relationship as an equation using the constant of variation k, substituting given values to find k, and then substituting values into the original equation to find the unknown. Examples include finding y when x is a given value for direct and inverse variation problems, and finding u when v and w are given values for a joint variation problem. The final example involves finding the number of vibrations per second of a guitar string when the tension and length are given values using direct and inverse variation.