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Simple rule of three

Calculate a missing value in direct proportionality.

Calculator

a must be non-zero.
Example: a units → b €
We calculate x corresponding to c.
Rounding

Results

x (value sought)10.00
ratio (b / a)2.50
Formula
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Examples

Click to prefill

Definition & method

In direct proportionality, the ratio b/a is constant.

Formula
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  1. Write the equality of the ratios: b/a = x/c.
  2. Isolate x: x = (b × c) / a.
  3. Check that a ≠ 0.
Example: a=10 corresponds to b=25, then c=4 corresponds to x=10.
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About: Simple rule of three

This tool allows you to quickly calculate a missing value using the simple rule of three, in a case of direct proportionality.

When to use this tool?

  • Immediate calculationSolve a proportionality calculation in a few seconds.
  • Homework and revisionsCheck a math exercise without redoing the entire reasoning.
  • Daily lifeAdjust a quantity, price, or duration proportionally.

Result obtained

The missing value calculated automatically.

A reliable calculation based on cross multiplication.

To be checked before use

  • The quantities must be proportional.
  • The units must be consistent.

Tip

If one quantity doubles, the other must also double: otherwise, it is not a direct proportionality.