The rule of three is used to calculate an unknown value when three values are known in a proportional relationship. If A/B = C/X, then X = (B × C) / A.

The rule of three is used when two quantities are proportional. Common examples: price per kilo, speed and time, recipes for different numbers of servings.

Set up the proportion: A relates to B as C relates to X. Multiply the known opposite values (B × C) and divide by the known value (A) to get X.