Rule of Three Calculator
Solve a simple rule of three by filling in three of the four values in the ratio A/B = C/X. The calculator finds the unknown value instantly.
How it works
The simple rule of three solves proportions of the form A/B = C/X. Fill in three of the four values and leave the one you want to find blank. The tool applies cross multiplication (A Γ X = B Γ C) and isolates the unknown value, detecting whether the quantities are directly or inversely proportional.
Facts about the Rule of Three Calculator
- Solves simple rule of three for directly or inversely proportional quantities.
- Lets you find any one of the four values in the ratio A/B = C/X.
- Uses cross multiplication and isolation of the unknown to compute the result.
- Warns automatically when the divisor entered is zero.
Official references
Check the official references to understand how the rule of three works:
- Juiz de Fora City Hall β Mathematics Module: Official booklet (public exam) defining the simple rule of three for direct/inverse proportions with step-by-step solutions.
- UFRGS β Proportions and Rule of Three: Definition of direct/inverse proportionality and the rule of three as a proportion y'/x' = y"/x".
- OBMEP β Ratios and Proportions Module: Official OBMEP/IMPA material (7th grade) on simple and compound rule of three, direct and inverse, with entrance exam exercises.