Proportional calculation.
The Rule of Three (also called Cross-Multiplication or Proportion Method) is one of the most useful mathematical techniques for everyday life. It solves problems where two quantities have a fixed relationship (direct proportion). From recipe scaling to currency conversion to technical calculations – it's universally applicable.
The method consists of three steps:
Step 1 (Known): A corresponds to B
Step 2 (Unit): 1 corresponds to B/A
Step 3 (Result): C corresponds to (B/A) × C = (B × C) / A
Formula: Result = (B × C) / A
Direct proportion: More of one means more of the other (e.g., more apples = more money). Inverse proportion: More of one means less of the other (e.g., more workers = less time). For inverse, multiply instead of divide: Result = (A × B) / C
Ask yourself: When one quantity increases, does the other increase or decrease? Price and quantity: direct. Workers and time: inverse. Speed and time: inverse. Fuel consumption and distance: direct.
Yes! The Rule of Three is the foundation of percentage calculations. Example: 100% equals $250, what is 15%? → (250 × 15) / 100 = $37.50. For percentage tasks, also try our Percentage Calculator.
Cross-multiplication is another name for the same technique. When you write A/B = C/X, you "cross multiply" to get A × X = B × C, then solve for X = (B × C) / A. It's the same formula as the Rule of Three.
Memory Tip: Write both ratios as fractions with the same units on top and bottom, then cross-multiply to solve!