What Is the Rule of Three?
The rule of three is a mathematical method for finding an unknown value from three known values that are related by proportionality.
When is it direct or inverse?
- Direct: when one quantity increases, the other also increases (price × quantity)
- Inverse: when one quantity increases, the other decreases (speed × time)
Formulas
Direct: A → B
C → x → x = (C × B) ÷ A
Inverse: A → B
C → x → x = (A × B) ÷ CHow do you identify the type?
Ask yourself: “If the first value doubles, does the second also double (direct) or drop by half (inverse)?”
Frequently asked questions
Can I use the rule of three to convert units?
Yes! Conversions are direct proportions. Example: 1 km = 1000 m → 3.5 km = ?. Apply the direct rule of three.
Does the rule of three work with percentages?
Yes! 100% corresponds to the total. If X% = Y, set up the proportion: 100 → total / X → Y.
What is the compound rule of three?
When the problem involves three or more quantities. For each quantity you determine the type (direct or inverse) and set up a single equation.
When should I NOT use the rule of three?
When the relation between the quantities is not linear (e.g. quadratic, logarithmic or exponential). Always check that proportionality really applies.