About this tool
The rule of three finds the fourth value of a proportion: if A corresponds to B, what corresponds to C? In a direct proportion both quantities grow together (more kilos, higher price) and X = B × C ÷ A. In an inverse proportion one grows while the other shrinks (more workers, fewer days) and X = A × B ÷ C. Type the three known values, choose the kind of proportion and the calculator shows X, the calculation and a check. It also works for percentages: if 100% is 30, what is 45%? Everything is calculated in your browser.
How to use it
- 1 Choose direct or inverse proportion.
- 2 Type A and B, the pair you know, and C, the value you want the match for.
- 3 Read X, the calculation step by step and the check.
- 4 Use the examples to see common cases: prices, work and percentages.
Frequently asked questions
How do I know if the proportion is direct or inverse?
Ask what happens to the second quantity when the first one doubles. If it also doubles (price of more kilos), it is direct. If it halves (days needed with more workers), it is inverse.
What is the formula?
Direct: X = B × C ÷ A. Inverse: X = A × B ÷ C. In both cases you multiply crosswise and divide by the remaining value.
Can I use it for percentages?
Yes. Put 100 in A and the total in B, then the percentage in C: X is that percentage of the total.
Guide
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