(The Rule of). A method of solving certain mathematical questions generally done by equations. Suppose the question is this: “What number is that whose half exceeds its third by 12?” Assume any number you like as the supposed answer—say 96. Then, by the question, 96 ÷ 2 = 96 i.e. 54, but 48 does not equal 54, the latter is 16 too much. Well, now state by rule of proportion thus, 16: 12:: 96 to the answer, which is 72, the number required.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894