Rule of Three
Solve proportion problems using the rule of three.
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What is rule of three?
Rule of three is a mathematical technique used to find a value proportional to another when three related values are known. It is widely used in percentage calculations, recipes, discounts, measurements, scales, and everyday problems. The basic formula is: X = (B × C) / A, where A is to B as C is to X.
Practical usage examples
- Shopping: If 2kg cost $10, how much do 5kg cost? (A=2, B=10, C=5 → X=25)
- Fuel: If a car travels 300km with 20L, how far with 1L? (A=20, B=300, C=1 → X=15)
- Recipes: If a recipe uses 2 eggs for 4 people, how many eggs for 6 people? (A=4, B=2, C=6 → X=3)
- Scales: If 1cm on a map represents 10km, how many km does 3cm represent? (A=1, B=10, C=3 → X=30)
- Discounts: If a $100 product has a 15% discount, what is the final value? (A=100, B=15, C=100 → X=15, then subtract)
How to use
- Identify the three known values of the proportion.
- Fill in fields A, B, and C with the known values.
- Click Calculate to find the unknown value.
- The result is displayed with up to 4 decimal places for greater precision.
Rule of three formula
The formula used by the tool is: X = (B × C) / A
A cannot be zero. If A is zero, the tool will display an error message.
Frequently asked questions
Does the tool work with negative values?
Yes, as long as the proportional relationship holds. The tool accepts negative and decimal values.
Can I use rule of three to calculate percentages?
Yes. Rule of three is very useful for percentage calculations. For example: if 100% equals $200, how much is 15%? (A=100, B=200, C=15 → X=30)
Do I need to know the formula to use the tool?
No. Just fill in the three known values (A, B, and C) and the tool automatically calculates X using the formula X = (B × C) / A.