When two ratios are equal, the four quantities composing them are said to be in proportion.  Thus

If\(\frac{a}{b} = \frac{c}{d}\), then \(a, b, c, d\) are in proportion.

Hence if any three terms of a proportion are given, the fourth may be found.   Thus if \(a, c, d\) are given, and then b can be found.

Conversely, if there are any four quantities, \(a, b, c, d\), such, that \(ad = bc\), then \(a, b, c, d\) are in proportions; \(a\) and \(d\) being the extremes, \(b\) and \(c\) the means; 

Quantities are said to be in continued proportion when the ratio of the first to second, equals  the ratio of the second to third, which in turn equals the ratio of third to the fourth. Continued proportion is also known as Geometric Progression.

For example \(A, B, C, D\) are in continued proportion or G.P, if \(A/B = B/C = C/D\)