What is a mathematical sentence that indicates a relationship between quantities that are not equivalent?

A mathematical sentence that indicates a relationship between quantities that are not equivalent is an inequality. Inequalities express the idea that one quantity is either less than, greater than, less than or equal to, or greater than or equal to another quantity. For example, the statement (x < 5) reveals that (x) is not equivalent to 5, as it is less than 5. This fundamental notion of non-equivalence is what sets inequalities apart from equations, where both sides of the statement are equal.

In contrast, proportions and equations deal with relationships of equivalence, where proportions set two ratios equal to each other, and equations indicate that two expressions have the same value. An expression, on the other hand, does not show a relationship between two quantities at all but instead represents a value or quantity. Therefore, the correct answer that describes a relationship indicating quantities that are not equivalent is indeed an inequality.