Proportions and rule of three
Solve direct and inverse proportions using the rule of three.
For a : b = c : x choose Direct. For inverse relationships use a × b = c × x.
Result
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Method, examples and limits
The direct formula is x = b × c ÷ a: if 2 items cost 10, then 5 cost 25. The inverse formula is x = a × b ÷ c: 2 people working 10 hours correspond to 5 people working 4 hours, assuming equal productivity. The divisor must be nonzero. Use a dot or comma for decimals, without thousands separators.
Frequently asked questions
When should I use inverse proportion?
When the product of the two quantities stays constant: the model uses a × b = c × x.
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