Key Idea: “Per”

When a problem uses “per,” it signals division.
The quantity before “per” goes on the top, and the quantity after “per” goes on the bottom.

General Rule:

Quantity before “per”÷Quantity after “per” 

Why this matters for the SAT:

SAT questions often use “per” in unit rates, speed, density, and similar contexts. Correctly identifying which value is the numerator and which is the denominator is key to setting up the right equation.

Key Idea: “Ratio”

A ratio is a comparison between two quantities. It’s usually expressed as a fraction or with a colon (:).
The first quantity corresponds to the numerator, and the second quantity corresponds to the denominator.

General Rule:

Ratio of A to B=A/B

Why this matters for the SAT:

Ratios appear in proportions, geometry, mixture problems, and data interpretation. Knowing which number comes first and how to set it up prevents inversion mistakes and makes proportion problems straightforward.