rule of three

American  

noun

Mathematics.

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

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rule of three British  

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

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Etymology

Origin of rule of three

First recorded in 1585–95

Example Sentences

She knew her mathematics to the Rule of Three, but whether she could cypher into fractions, I had no idea.

From Literature

He has a rule of Three Ps “is it possible, plausible or probable?” to judge the merits of the argument.

From MarketWatch

But you’ve probably heard of the “rule of three.”

From Salon

So, my wife and I vowed to implement the rule of three.

From Salon

It’s tentatively called “The Rule of Three,” and just like “We the Animals,” it tells the story of three brothers.

From Los Angeles Times