Ratios and proportions can be used to compare quantities. A proportion exists when two ratios are equivalent, meaning the quantities have an equal relationship. To check if two ratios form a proportion, you can use cross multiplication or the scale factor method. Cross multiplication checks if the products of the means and extremes are equal. The scale factor method identifies a common factor that makes the ratios equal when applied. Homework involves copying proportions and showing work checking for equivalency using these methods.