rule of three

noun Mathematics.

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

Origin of rule of three

First recorded in 1585–95

Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2018

Examples from the Web for rule of three

Historical Examples of rule of three

It is as if we were to talk of the colour of sound, or the longitude of the rule-of-three.
Short Studies on Great Subjects
James Anthony Froude
And I was by no means one to puzzle it out, as I would a sum in the rule-of-three.
The Atlantic Monthly, Vol. 13, No. 79, May, 1864
Various
I do not require to examine on this point, as I consider it to be a rule-of-three calculation.
Diary in America, Series Two
Frederick Marryat (AKA Captain Marryat)
He got into the rule-of-three without having learned numeration, addition, subtraction, and division.
Homes and haunts of the most eminent British poets, Vol. II (of 2)
William Howitt
When it was found that he could not do decimals, he was put back to the rule-of-three, and then pronounced incurable.
Homes and haunts of the most eminent British poets, Vol. II (of 2)
William Howitt

British Dictionary definitions for rule of three

rule of three

noun

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

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