Final answer:
Proportional reasoning is used when one quantity is a constant multiple of the other, maintaining the same ratio even as the quantities change.
Step-by-step explanation:
You can use proportional reasoning to solve a problem when one quantity is a constant multiple of the other. This means that as one quantity changes, the other changes in a way that keeps the ratio the same. For instance, this occurs in scaling problems, where objects are increased or decreased in size but maintain their shape. When dealing with ratios and proportional relationships, it's important to identify if there is a direct proportionality (both quantities increase or decrease together) or an inverse proportionality (one increases while the other decreases).