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The rule of three finds the fourth value when three are known and two quantities scale together: if a gives b, then c gives b × c ÷ a. It is the everyday tool for unit pricing and scaling quantities up or down.
This assumes the two quantities rise and fall together. Inverse relationships — where more of one means less of the other — need the values arranged differently.
That is the promise behind every tool here. Files are read in your browser tab, processed there, and discarded when you close it — never uploaded to our servers.
