An expression where two ratios are compared and these two ratios are set equal to each other is known as proportion. Proportion is a relationship between four numbers in which the first ratio equals second ratio. It is represented by fractions or by using colon.

For example: $\frac{a}{b}$ $=$ $\frac{c}{d}$ or $a : b :: c : d$

It is read as "a is to b as c is to d."  These four terms are called as the terms of proportion.

Here, a and d are called the extremes while b and c are called means.
Unknown value can be calculated by the following steps:

Step 1:  Observe the given proportion
               $\frac{a}{b}$ $=$ $\frac{c}{d}$
Step 2:  Do cross multiplication and simplify.
                  $ad = bc$
Step 3: To check whether it is valid proportion or not. The product of the extremes should be equal to the product of the means.