Rule of three calculator: direct and inverse proportions
The rule of three is one of the most useful mathematical procedures in everyday life. It allows finding the unknown fourth value when three proportionally related values are known.
Direct rule of three
Used when quantities are directly proportional: as one increases, the other increases in the same proportion. Formula: x = (B × C) / A. Example: if 5 kg cost $10, how much do 8 kg cost? x = (8 × 10) / 5 = $16.
Inverse rule of three
Used when quantities are inversely proportional: as one increases, the other decreases. Formula: x = (A × B) / C. Example: if 4 workers do a job in 6 days, how many days will 8 workers take? x = (4 × 6) / 8 = 3 days.
Practical applications
- Currency and exchange rate conversions.
- Scales on maps and architectural plans.
- Calculating proportions in cooking recipes.
- Speed, time, and distance in physics.
- Mixtures and concentrations in chemistry.
- Discounts and margins in business.