Given four rational numbers a, b, c, d, nonzero, in that order, we say that they form a ratio when the ratio of the 1st to the 2nd equals the ratio of the 3rd to the 4th.

Like this:

or *a: b = c: d*

*The*is for

*B*as

*ç*is for

*d*")

The numbers *a, b, c* and *d* are the terms of the proportion, where:

*B*and*ç*the**means**of the proportion.*The*and*d*the**extremes**of the proportion.

Example:

Given the proportion we have:

Reading: 3 stands for 4 just as 27 stands for 36.*Means:* 4 and 27 *Extremes:* 3 and 36