Math
Rule of Three Calculator
Solve simple proportions in seconds.
Result
28.0000
If 3 → 12, then 7 → X
Download results
Quick summary
Solve a proportion of the form A : B = C : X, the classic rule of three used in everyday math, cooking, and conversions.
How to calculate rule of three manually
- Cross-multiply: X = (B × C) / A.
- The rule assumes a direct proportion between A→B and C→X.
Solve a proportion of the form A : B = C : X, the classic rule of three used in everyday math, cooking, and conversions.
How it works
Cross-multiply: X = (B × C) / A. The rule assumes a direct proportion between A→B and C→X.
Example
If 3 → 12, then 7 → 28 (since 12 × 7 / 3 = 28).
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