Price by weight
- Input
- A=3, B=9, C=5
- Expected output
- X = 15
5 kg at $3/kg = $15.
direct rule of three
Direct Rule of Three
In a direct proportion, when one quantity increases, the other increases at the same rate. X = B × C / A.
What is a direct proportion
- Two quantities are directly proportional when their ratio is constant: A/B = C/X. Cross-multiplying: A × X = B × C, so X = B × C / A.
- Example: if 3 kg of apples cost $9, how much do 5 kg cost? A=3, B=9, C=5 → X = 9 × 5 / 3 = $15.
Examples
km to m conversion
- Input
- A=1, B=1000, C=3.5
- Expected output
- X = 3500
3.5 km = 3500 m.
Full tool FAQ
In a direct proportion, two quantities grow or shrink together. If A is to B as C is to X, then X = B × C / A. Example: 3 workers produce 6 items per day; 5 workers would produce 10.
Frequently asked questions
How do I tell direct from inverse?
If quantities grow together, it is direct. If one grows and the other shrinks, it is inverse.
Does this page replace official or professional review?
No. It helps explain the scenario and use the tool more safely, but real decisions should consider official sources, full context and qualified guidance when needed.