Proportion describes how two quantities change together. A unit rate is the amount for 1 unit, such as cost per apple or cartons per hour, and it often helps you spot the relationship.

In direct proportion, both quantities scale by the same factor, so if apples double, the cost doubles. The algebra model is $y=kx$, where $k$ is the constant of proportionality.

In inverse proportion, increasing one quantity makes the other decrease by the same factor, so if workers double, the time halves. The algebra model is $y=\frac{k}{x}$.