# rule of three

noun, Mathematics.
1.
the method of finding the fourth term in a proportion when three terms are given.
Origin of rule of three
1585-1595
1585-95
Dictionary.com Unabridged
Based on the Random House Dictionary, © Random House, Inc. 2016.
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Examples from the Web for rule-of-three
Historical Examples
• It is as if we were to talk of the colour of sound, or the longitude of the rule-of-three.

James Anthony Froude
• I do not require to examine on this point, as I consider it to be a rule-of-three calculation.

Frederick Marryat (AKA Captain Marryat)
• And I was by no means one to puzzle it out, as I would a sum in the rule-of-three.

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

• 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!

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

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

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

James Anthony Froude, Edward A. Freeman, William Ewart Gladstone, John Henry Newman and Leslie Stephen
British Dictionary definitions for rule-of-three

## 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
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition
Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012
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### Difficulty index for rule of three

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### Word Value for rule

4
6
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