A proportion says two ratios are equal. If 2/4 = 3/6, that is a proportion. Cross-multiplying gives 2 × 6 = 4 × 3, both equal to 12. Proportions are used to solve for unknown quantities in similar figures, scale drawings, and unit conversions.

Proportions express relationships. If a recipe uses 2 cups of flour for 12 cookies, how much for 30 cookies? Set up 2/12 = x/30, cross-multiply, and solve: x = 5. The method is simple and widely useful.

In geometry, similar triangles have proportional sides. In physics, quantities like speed and distance are proportional at constant rates. In statistics, proportions describe parts of a whole. The concept connects arithmetic, algebra, and geometry.

• Equation of two equal ratios
• Solved by cross-multiplication
• Used in scaling, recipes, and similar figures
• Connects arithmetic, algebra, and geometry