The so-called rule of three is an educational tool utilized historically to verbalize the process of solving basic linear equations with four terms where three of the terms are known. Evidence suggests that there are two "different" rules of three: One direct and one inverse.
The direct rule of three describes the process for finding the fourth term 
of a proportion
 |
(1)
|
given that 
,
, and 
are known. In words, the solution is to "multiply the
second and third numbers together, and divide 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 
, 
,
and 
are known. In words, one solves the problem
by "multiplying the first and second terms together, and dividing the product
by the third" so that
 |
(3)
|
