What you need to know
You can only add or subtract fractions when the pieces are the same size — that is, when the denominators are the same. If they are different, you first rename one or both fractions so they share a common denominator. Then you add or subtract the numerators and keep the denominator. Finally, simplify the answer if you can.
- Find the lowest common denominator (LCD): the smallest number both denominators divide into.
- Rename each fraction with that denominator (multiply top and bottom by the same number).
- Add or subtract the numerators. Keep the denominator.
- Simplify, and change an improper fraction to a mixed number if needed.
12 + 13
- LCD of 2 and 3 is 6.
- 12 = 36 and 13 = 26.
- 36 + 26 = [frac 5/6].
34 − 16
- LCD of 4 and 6 is 12 (multiples of 4: 4, 8, 12; multiples of 6: 6, 12).
- 34 = 912 and 16 = 212.
- 912 − 212 = [frac 7/12].
25 + 310
- 10 is a multiple of 5, so the LCD is 10 and only one fraction needs renaming.
- 25 = 410.
- 410 + 310 = [frac 7/10].
56 + 34 (answer bigger than 1)
- LCD of 6 and 4 is 12.
- 56 = 1012 and 34 = 912.
- 1012 + 912 = 1912.
- 12 twelfths make 1 whole, with 7 twelfths left over: 1[frac 7/12].
| Denominators | Multiples | LCD |
|---|
| 4 and 6 | 4, 8, 12 … and 6, 12 … | 12 |
| 3 and 5 | 3, 6, 9, 12, 15 … and 5, 10, 15 … | 15 |
| 2 and 8 | 8 is already a multiple of 2 | 8 |
| 6 and 9 | 6, 12, 18 … and 9, 18 … | 18 |
Common mistake: adding the denominators. 12 + 13 is not 25 — two fifths is less than a half, so that answer cannot be right. The denominator only tells you the size of the pieces.
Multiplying the two denominators always gives a common denominator, but not always the lowest one. 4 × 6 = 24 works for quarters and sixths, but 12 is smaller and means less simplifying at the end.
112 + 214
- Add the whole numbers: 1 + 2 = 3.
- Add the fractions: 12 = 24, so 24 + 14 = 34.
- Answer: 3[frac 3/4].
Check yourself
- 14 + 18 = ?
- 23 − 14 = ?
- 13 + 25 = ?
- 56 − 12 = ? (simplify)
- 34 + 23 = ? (as a mixed number)
- 113 + 116 = ?
Answers: 1. 38 2. 512 3. 1115 4. 26 = 13 5. 1712 = 1512 6. 236 = 212