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Rule of three calculator for free
If 3 kg cost 6, 5 kg cost 10: that is a direct rule of three. If 4 people take 6 days, 8 people take 3, and that is an inverse one. It also solves compound problems with up to four quantities, and always explains every step.
- Free with an account
- Direct, inverse and compound
- Step by step
- Choose the type
- Type the values
- Read the result and the steps
HOW TO USE IT
Three steps.
Choose the type
Direct if both amounts grow together; inverse if one grows as the other falls; compound if several quantities change.
Type the values
In the simple one, A goes with B and you want what goes with C. In the compound one, type each quantity before and after.
Read the result and the steps
You get X with up to 4 decimal places, the formula used and a step-by-step explanation.
NEXT STEP
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Rule of three: common questions
How do I know if it is direct or inverse?
Think about what happens if one amount doubles. If the other doubles too (more kilos, higher price), it is direct. If it halves (more workers, fewer days), it is inverse.
What is the rule of three formula?
Direct: X = B × C ÷ A. Inverse: X = A × B ÷ C. In a compound problem, the known value is multiplied by (new ÷ old) for each direct quantity and by (old ÷ new) for each inverse one.
Does it work for any problem?
Only when the relationship is proportional. If a price has a fixed part, or the output changes with the amount, the rule of three gives an approximate or wrong answer.
Is my data sent to a server?
No. The numbers, dates and names you type are calculated in your browser and are not sent to Cocuyo or to any third party.
Is it really free?
Yes. You can try it before signing up; to keep using it, we ask for a free account, with no subscription. It uses no credits and does not count toward your 30 welcome uses: it has no usage cap.
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