R2 · Lesson 4: Add & Subtract Fractions

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Why this matters for Grade 6: Fraction addition and subtraction appear throughout Grade 6 — in equations, ratios, and measurement. Mastering the LCD method here also gives you the exact tool needed to rewrite fractions before dividing them (6.NS.1).

Learn It

Add & Subtract Fractions

Like denominators (same bottom number): just add or subtract the numerators; keep the denominator the same. Simplify if needed.

Example 1 — Like denominators
1/4 + 2/4 = (1+2)/4 = 3/4
3/4 − 1/4 = (3−1)/4 = 2/4 = 1/2 (simplified)

1/4 + 2/4 = 3/4 (3 parts out of 4)

Unlike denominators: find the LCD (Least Common Denominator), rewrite both fractions with that denominator, then add or subtract.

Example 2 — Unlike denominators
1/2 + 1/3 = ?
LCD of 2 and 3 = 6
1/2 = 3/6 | 1/3 = 2/6
3/6 + 2/6 = 5/6 → Answer: 5/6

5/6 − 1/3 = ?
LCD of 6 and 3 = 6
5/6 = 5/6 | 1/3 = 2/6
5/6 − 2/6 = 3/6 = 1/2 (simplified)

Try It · Guided

Work These Together

1. What is 2/5 + 1/5?

Hint

Same denominator: add numerators. 2 + 1 = 3. Keep the 5. Answer: 3/5.

2. What is 1/4 + 1/2? (Simplify your answer.)

Hint

LCD of 4 and 2 is 4. 1/2 = 2/4. Now add: 1/4 + 2/4 = 3/4.

Practice · On Your Own

You Try

3. What is 5/8 − 3/8? (Give your answer in simplest form.)

Hint

Same denominator: subtract numerators. 5 − 3 = 2. Answer: 2/8. Simplify by dividing both by 2: 1/4.

4. What is 2/3 + 1/6? (Simplify your answer.)

Hint

LCD of 3 and 6 is 6. 2/3 = 4/6. Now add: 4/6 + 1/6 = 5/6. Already in simplest form.

5. What is 3/4 − 1/3? (Simplify your answer.)

Hint

LCD of 4 and 3 is 12. 3/4 = 9/12. 1/3 = 4/12. 9/12 − 4/12 = 5/12. Already in simplest form.

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Exit Ticket

Show What You Know

6. What is 1/2 + 1/3?

Hint

LCD of 2 and 3 is 6. 1/2 = 3/6. 1/3 = 2/6. 3/6 + 2/6 = 5/6.

7. What is 7/8 − 1/2? (Give your answer in simplest form.)

Hint

LCD of 8 and 2 is 8. 1/2 = 4/8. 7/8 − 4/8 = 3/8. GCF(3,8) = 1, so 3/8 is already simplest.

← Lesson 3: Compare & Order Next: Multiply Fractions →