Rule of Three Calculator
Calculate unknown values in proportions using the rule of three (cross multiplication).
How it works
What is the Rule of Three?
The rule of three is a mathematical method used to solve problems involving proportions. It helps you find an unknown value when you know three related values. The rule states: if A is to B as C is to X, then X = (B × C) / A.
How to Use the Rule of Three Calculator
Our calculator makes it easy to solve rule of three problems. Enter three known values (A, B, and C) and the calculator will find the unknown value (X).
Example 1: Recipe Scaling
If 2 cups of flour make 12 cookies, how many cookies can you make with 5 cups of flour? Enter A=2, B=12, C=5. The result is X=30 cookies.
Example 2: Speed and Distance
If a car travels 120 km in 2 hours, how far will it travel in 5 hours? Enter A=2, B=120, C=5. The result is X=300 km.
The Formula
The rule of three formula is:
X = (B × C) / A
Where:
- A and B are the first pair of values
- C is the known value from the second pair
- X is the unknown value we want to find
Common Applications
The rule of three is useful in many real-world scenarios:
- Cooking: Scaling recipes up or down
- Shopping: Comparing prices and quantities
- Travel: Calculating time, distance, and speed
- Business: Determining costs and profits
- Construction: Estimating materials needed
Important Notes
Remember that value A cannot be zero, as division by zero is undefined. Always ensure your inputs are positive numbers for meaningful results. The rule of three works best when there is a direct proportional relationship between the values.