Solve proportions with the rule of three: enter three values and the fourth is calculated as you type. Covers direct and inverse proportionality, shows the formula with your numbers plugged in, and explains the result in a sentence.
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X = 2 × 7 ÷ 3 = 4.67
If 3 apples correspond to 2 €, then 7 apples correspond to 4.67 €.
This tool solves proportions with the rule of three. Fill in three of the four values in the sentence and the fourth is calculated as you type, whichever one you leave out. It handles both direct proportionality, where the values grow together, and inverse proportionality, where one shrinks as the other grows.
The formula is always shown with your numbers plugged in, so you can see exactly how the answer was reached, and the result is also written out as a plain sentence. Optional unit labels like apples and € make that sentence read naturally.
Direct proportionality means the two quantities move together. If 3 apples cost 2 €, then 7 apples cost more, and the tool computes X = B × C ÷ A. Prices, distances and recipe amounts usually work this way.
Inverse proportionality means one quantity shrinks as the other grows. If 4 workers need 6 hours, then 8 workers need less time, not more, and the formula becomes X = A × B ÷ C. Workers and time, speed and travel time, or pump count and fill time are typical inverse cases. If your answer looks upside down, you probably need the other mode.
The rule of three is cross multiplication in disguise. In a direct proportion A : B = C : X the products across the equals sign match, so A × X = B × C, and dividing by A gives X. In an inverse proportion the products of each pair match instead, so A × B = C × X. The tool picks the right rearrangement for whichever value you left empty and shows it with your numbers, so you can follow the math or copy it into homework.
Some questions have no answer. When the value you solve for would require dividing by zero, the tool says so in plain words instead of showing an error code. For percentage questions like discounts and tips, the Percentage Calculator asks them directly, and for switching between measurement units there is the Unit Converter.
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