E. Cobham Brewer 1810–1897. Dictionary of Phrase and Fable. 1898.

False (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 + 3 + 12, or 48 = 32 + 12, i.e. 54, but 48 does not equal 54, the latter is 16 too much.

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Well, now state by rule of proportion thus, 16: 12: : 96 to the answer which is 72, the number required.