The distinction between part-part and part-whole relationships is seldom discussed. Usually we refer to part-part relationships as ratios rather than fractions. The ratio representation is a better way to write the relation, because you won't be tempted to use the rules of fractions to work with them (for example, a ratio of 3 parts to 0 parts cannot be written as a fraction). However, the part-part relationship is sometimes written in fraction form (as it is in this section), even though it represents a ratio. So be sure to understand the difference.
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