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Rule of Three

Solve rule of three problems.

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The Golden Rule of Math

The Rule of Three is a mathematical method used to solve problems involving proportions. If you know three numbers, you can find the fourth.

A black and white cartoon of a balance scale, showing two sides in perfect equilibrium.A black and white cartoon of a balance scale, showing two sides in perfect equilibrium.

Direct vs. Inverse

Direct Proportion

As one value increases, the other increases too. (e.g., buying more apples costs more money).

A / B = C / X

Inverse Proportion

As one value increases, the other decreases. (e.g., more workers means less time to finish a job).

A * B = C * X

Everyday Examples

  • Cooking: If a recipe for 4 people needs 200g of flour, how much do you need for 6 people?
  • Travel: If you travel 100km in 1 hour, how far will you go in 2.5 hours?
  • Currency: If 1 USD is 0.9 EUR, how many EUR is 50 USD?
A black and white cartoon of a chef looking at a recipe book and adjusting the ingredients for a larger pot.A black and white cartoon of a chef looking at a recipe book and adjusting the ingredients for a larger pot.

Learn More

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