Rule of three
Solve a direct proportion: if A is to B, find X from C.
If A corresponds to B, what does C correspond to?
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How it works
The direct rule of three finds a fourth value from three known ones when two quantities are proportional: if one doubles, so does the other. It underpins scaling recipes, prices, and consumption.
Formula
if A is to B, then C is to X → X = B × C ÷ A
Example
If 3 kg of apples cost €7.50, then 5 kg cost 7.50 × 5 ÷ 3 = €12.50.
Frequently asked questions
What is an inverse rule of three?
One where raising one quantity lowers the other, like workers and days of work. This tool solves the direct version.
What if A is zero?
You cannot divide by zero, so A must be non-zero.