Roman Numerals: Symbols and the Two Big Rules
3-5
1
Least Common Denominator (LCD): Finding a Common Denominator
Lesson Details
Grade Level
6-8
Duration (minutes)
1
Summary
Grade 6-8 students learn how to find the Least Common Denominator (LCD) before adding fractions with unlike denominators. Using the example of 1/4 and 1/6, the video shows why mismatched denominators can't be added directly, then walks through listing multiples to find the LCD (12), rewriting each fraction, and adding to reach 5/12.
Student Worksheet: Least Common Denominator (LCD)
Teacher Note: Students can watch the video lesson above and answer the questions. Click the “Teacher Answer Key” dropdown below when you are ready to review the solutions!
Key Concepts:
• Why it matters: Denominators must match before you can add fractions — the pieces have to be the same size.
• LCD: The Least Common Denominator is the least common multiple of the denominators.
• The steps: List multiples of each denominator, find the first one they share, rewrite each fraction with that denominator, then add.
Instructions: Read each question carefully and write your answer in the space provided.
- Question 1: Why can’t you add 1/4 + 1/6 right away?
- Question 2: What is the LCD, in your own words?
- Question 3: List the first few multiples of 4 and of 6. What is the LCD of 4 and 6?
- Question 4: Rewrite 1/4 and 1/6 both with a denominator of 12.
- Question 5: Add 3/12 + 2/12. What is the answer?
- Question 6: Find the LCD of 3 and 5, then rewrite 1/3 and 1/5 using that denominator.
Click to reveal Teacher Answer Key
1. The pieces are different sizes (fourths vs. sixths), so they can’t be combined until the denominators match.
2. The LCD is the least common multiple of the denominators — the smallest number both denominators divide into evenly.
3. Multiples of 4: 4, 8, 12. Multiples of 6: 6, 12. The first shared number is 12, so the LCD is 12.
4. 1/4 x 3/3 = 3/12. 1/6 x 2/2 = 2/12.
5. 3/12 + 2/12 = 5/12.
6. LCD of 3 and 5 is 15. 1/3 x 5/5 = 5/15. 1/5 x 3/3 = 3/15.
Disclaimer: This worksheet was generated with AI assistance based on original video content. It is intended for educational reference only. Teachers are encouraged to review and adjust questions to fit their curriculum standards before classroom use.
🎯 Extension Activity: Give students 3 pairs of fractions with different denominators (like 1/2 and 1/5, or 2/3 and 1/4). Have them find the LCD for each pair, rewrite both fractions, and add them together.