Glossary
ratio
A ratio compares two quantities, written a : b, and it stays the same when both sides are multiplied or divided by the same number.
A ratio compares two quantities — 2 ml of solution to 5 ml of water — and is written with a colon, as a fraction, or with the word to. Two properties do the work. The order is part of the meaning, so 2 : 5 is not 5 : 2, which is why every ratio question starts with which one is named first. And the ratio is a relationship, not an amount: multiply or divide both sides by the same number and the ratio is unchanged, which is how a ratio scales to a total. Keep the parts and the whole apart — in a 1 : 4 mixture the drug is one part out of five, and a question asking for the fraction of the total wants one fifth, not one fourth. Set the ratio up with its units written beside the numbers and scale it in the same direction on both sides.
Taught in
Related terms
- proportion — A proportion is a statement that two ratios are equal, with one of the four values missing — and the missing value is what the calculation returns.
- unit rate — A unit rate is a rate reduced to one unit — dollars per item, kilometers per hour, milligrams per milliliter — which is what makes two offers comparable.
- cross multiplication — Cross multiplication solves a proportion by multiplying each numerator by the opposite denominator, then dividing by the remaining number.
Source: The math guides in this course (ratio and proportion, measurement and conversion, dosage calculation): every definition and worked value above was written for this course, because arithmetic and unit conversion are not a publisher's prose, and each guide shows the method worked. Elsevier's own HESI A2 page, which names math among the areas the exam covers, is the exam-side source. (Elsevier HESI A2 page accessed 2026-10-23; definitions written 2026-10-27)