Ratios / Introduction
Introduction to Ratios
Compare two quantities using ratio notation. Write equivalent ratios. Understand part-to-part and part-to-whole. Grade 4.
What is a Ratio?
A ratio compares two quantities. Both quantities must be in the same units (or dimensionless).
Example: In a class of 10 boys and 15 girls
- Ratio of boys to girls: 10:15 (simplify → 2:3)
- Ratio of girls to boys: 15:10 → 3:2
- Ratio of boys to total: 10:25 → 2:5 (part to whole)
Three Ways to Write a Ratio
a : b Colon notation (most common in maths) a to b Word form a / b Fraction form (useful for calculations) Simplifying Ratios
Divide both numbers by their GCF (greatest common factor).
12 : 8
GCF(12, 8) = 4
÷4 → 3 : 2 ✓
Equivalent Ratios
Multiply (or divide) both parts of a ratio by the same number to get an equivalent ratio — just like equivalent fractions.
2 : 3
×2 → 4 : 6
×3 → 6 : 9
×5 → 10 : 15
All equivalent to 2:3
Practice Drills
20
Example: 4 × 6