Proportions are usually written in one of these two ways: a : b = c : d or a / b = c / d in which a, b, c, and d are real numbers and b 0 and d 0.

In either of the above two forms, the terms b and c are called the means of the proportion and the terms a and d are called the extremes. The multiplication property of equality shows that in any proportion the product of the means is equal to the product of the extremes: ad = bc.

If only one term of the proportion is a variable, then its value can be found by solving the proportion.