Rule of Three Calculator
Enter three known values to calculate the fourth using a direct or inverse proportion.
Espacio publicitario
- X
- 19.2
Espacio publicitario
How the rule of three works
In a direct rule of three, if A corresponds to B, then C corresponds to X, and both magnitudes grow in the same proportion.
Direct: X = (B × C) ÷ A
In an inverse rule of three, when one magnitude increases the other decreases in the same proportion.
Inverse: X = (A × B) ÷ C
Worked example
If 5 workers take 12 days to finish a job, how long would 8 workers take? It's inverse: X = (5 × 12) ÷ 8 = 7.5 days.
Frequently asked questions
- When is the direct rule of three used?
- When both magnitudes increase or decrease together, for example price and quantity of a product.
- When is the inverse rule of three used?
- When increasing one magnitude decreases the other, for example number of workers and time needed.
- Can I use decimals?
- Yes, all fields accept decimal values.
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