Seeing a Proportion

A ratio compares two quantities: 3 boys to 5 girls is the ratio 3:5. A proportion says two ratios are equal: 3/5 = 6/10.

Ratios run through daily life. Map scales (1 inch = 50 miles), recipe adjustments (doubling a recipe doubles every ingredient), and currency rates (1 dollar = 0.92 euros) are all ratios. Cross-multiplication solves any proportion: if a/b = c/d, then a × d = b × c. That single move handles unit conversion, similar triangles, and percentage problems — and, as you'll see, every home estimate in this lesson.