Rule of three calculator
Direct proportion (the more, the more) and inverse proportion (the more, the less), with result and working.
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Direct proportion
Example: 3 kg cost 12 francs. What do 5 kg cost?
Inverse proportion
Example: 4 people need 6 days. How long do 3 people need?
Frequently asked questions
When direct, when inverse?
Direct when both quantities grow together: double the amount, double the price. Inverse when one grows and the other shrinks: twice as many people, half as many days.
How do you work out the rule of three by hand?
First work out the value for 1, then for the amount you want: 3 kg cost 12, 1 kg costs 12 ÷ 3 = 4, 5 kg cost 4 × 5 = 20. With the inverse rule of three you multiply first and then divide.
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