The rule of three is a historical method for solving basic linear equations with four terms, of which three are known. There are direct and inverse versions.
The direct rule of three describes the process for finding the fourth term 
of a proportion
 |
(1)
|
given that 
,
, and 
are known and nonzero. In words,
the solution is to "multiply the second and
third numbers together, and divide
the product by the first" so that the term 
in equation
(1) above becomes
 |
(2)
|
The inverse rule of three describes the process of solving for 
when 
, 
, 
,
and 
are known, and 
. In words, one solves the problem by "multiplying
the first and second terms together, and dividing the
product by the third" so that
 |
(3)
|
