rule of three

noun

, Mathematics.

1. the method of finding the fourth term in a proportion when three terms are given.

rule of three

noun

1. a mathematical rule asserting that the value of one unknown quantity in a proportion is found by multiplying the denominator of each ratio by the numerator of the other

Word History and Origins

Origin of rule of three¹

First recorded in 1585–95

Example Sentences

Boys were never taught the rule-of-three until after they had learnt addition.

From Project Gutenberg

It is as if we were to talk of the color of sound, or the longitude of the Rule-of-three.

From Project Gutenberg

A man who's seen Injin fightin' don't go much on this here West Point fightin' by rule-of-three—but that ain't here or there!

From Project Gutenberg

He got into the rule-of-three without having learned numeration, addition, subtraction, and division.

From Project Gutenberg

When it was found that he could not do decimals, he was put back to the rule-of-three, and then pronounced incurable.

From Project Gutenberg