Rule of Three

The rule of three is a proportionality method. If a value A corresponds to B, then to find what corresponds to C, apply the formula: X = (B x C) / A.

Example 1: scaling a recipe

A recipe for 4 people requires 300 g of flour. For 6 people: X = (300 x 6) / 4 = 450 g of flour.

Example 2: currency conversion

If 1 EUR equals 1.08 USD, how much are 250 EUR? X = (1.08 x 250) / 1 = 270 USD.

Example 3: speed, distance, time

A car covers 90 km in 1 hour. In 2 hours and 30 minutes (2.5 h), it covers: X = (90 x 2.5) / 1 = 225 km. This method assumes a linear relationship between quantities. It does not apply to inversely proportional relationships without adjusting the formula.