Solving proportions

Math

Definition

Finding a missing value in a proportion (an equation stating that two ratios are equal). The main technique is cross-multiplication: if a/b = c/d, then ad = bc.

How It Works

  1. Set up the proportion with the unknown variable.
  2. Cross-multiply to eliminate the fractions.
  3. Solve the resulting equation for the variable.
  4. Check the answer by substituting back into the original proportion.

Examples

  • 3/4 = x/20 → 4x = 60 → x = 15
  • If a recipe for 4 servings needs 6 cups of flour, then 10 servings need x: 6/4 = x/10 → x = 15 cups
Key Fact

a/b = c/d → ad = bc

Related Terms

Writing ratios Unit rates Similar polygons Proportions in triangles

Study This Concept

Practice solving proportions with free review games in these units:

Pre-Algebra Unit 3: Ratios and Proportions