# ratio

A ratio is a rational number of the form *a*/*b* where *a* is called the **numerator** and *b* is called the **denominator**. Ratios may be expressed in the
form *a*:*b;* as fractions, *a*/*b*; or as percentages, (100*a*/*b*)%. Ratios
are usually reduced to their lowest terms: e.g., 15:3 = 5:1.

In order to possess a ratio, two quantities must be either pure numbers or measures of the same type, e.g.

20:5 = 20/5 = 4; 15 kg:3 kg = 5.

The value of the ratio (proportionality factor) is always a pure number.