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, (100a/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.