Solve simple direct and inverse rule of three problems with the step-by-step formula
Direct: if one quantity increases, so does the other (more pounds, higher price).
Solve any simple rule of three problem in seconds. Enter the three known values and the calculator finds the fourth, showing the formula it uses. It works with the direct rule of three (if one quantity increases, so does the other) and the inverse (if one increases, the other decreases). It's ideal for recipes, prices, proportions, speeds, mixtures and math homework.
You only need three values:
Choose whether the relationship is direct or inverse.
Enter the first pair of known values: A corresponds to B.
Enter the third value, C, whose counterpart you want to find.
The result X appears instantly along with the formula used.
If A corresponds to B, C corresponds to X = (B × C) ÷ A. It's used when both quantities grow together: more pounds, higher price.
If A corresponds to B, C corresponds to X = (A × B) ÷ C. It's used when one quantity grows and the other decreases: more workers, fewer days.
You can use decimals with a comma or a period, and negative numbers. The divisor (A in the direct rule, C in the inverse) cannot be zero.
The rule of three solves many everyday problems:
Shopping: if 3 kg cost 12, how much do 5 kg cost? (direct: 20).
Recipes: scale the quantities of a recipe for 4 people up to 6 people.
Work: if 4 people take 6 days, how long do 8 people take? (inverse: 3 days).
Scales and maps: convert map distances to real distances.
Ask yourself: if one quantity doubles, does the other also double? If so, it's direct; if it gets cut in half, it's inverse.
Always put quantities of the same type in the same column (kilos with kilos, price with price).
Use the same units: don't mix grams with kilos or minutes with hours.
For percentages, it's faster to use the Percentage Calculator.
It's a method for finding an unknown value from three known values that are in proportion. It can be direct or inverse.
If increasing one quantity also increases the other, it's direct. If increasing one decreases the other, it's inverse.
X = (B × C) ÷ A. For example, if 2 notebooks cost 10, 5 notebooks cost (10 × 5) ÷ 2 = 25.
X = (A × B) ÷ C. For example, if 4 workers take 6 days, 8 workers take (4 × 6) ÷ 8 = 3 days.
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