Rule of Three Calculator
Solve proportions easily: if A gives B, then C gives what? Uses cross multiplication to find the unknown fourth value.
Result
Rule of Three (cross multiplication) solves proportions when three values are known and the fourth is unknown:
If A → B, then C → ?
? = (B × C) / A
This works because of the proportion: A/B = C/?
Examples:
-
If 3 apples cost $2, how much do 10 apples cost?
- (2 × 10) / 3 = $6.67
-
If a car uses 5 gallons per 150 miles, how much fuel for 400 miles?
- (5 × 400) / 150 = 13.3 gallons
-
If a recipe for 4 servings needs 2 cups flour, how much for 10 servings?
- (2 × 10) / 4 = 5 cups
Inverse Rule of Three:
When values are inversely proportional (more workers = less time):
? = (A × B) / C
- If 4 workers take 6 days, how long for 8 workers?
- (4 × 6) / 8 = 3 days