Rule of Three Calculator
The rule of three finds an unknown value from three known ones that are in proportion. Enter A, B and C — "A is to B as C is to X" — and it solves X, in direct or inverse proportion.
Your numbers
Result
- Proportion
- Directly proportional
| Step | Detail |
|---|---|
| Setup | 3 is to 6 as 5 is to X |
| Type | Directly proportional |
| Formula | X = (6 x 5) / 3 |
| Result | X = 10 |
Direct rule of three
Set it up as A is to B as C is to X. In direct proportion you cross multiply: X = (B x C) / A. Example: if 3 apples cost $6, how much do 5 cost? X = (6 x 5) / 3 = $10. When both quantities grow together, the proportion is direct.
Inverse rule of three
Some quantities are inversely proportional: as one rises, the other falls. More workers, less time. Then you keep the same row instead of crossing: X = (A x B) / C. If 4 workers take 6 days, 8 workers take (4 x 6) / 8 = 3 days.
Frequently asked questions
Sources
- Ratios and proportions (rule of three)
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