When neither denominator is a multiple of the other, find a number both go into. Multiplying the two denominators always works: for 2/3 + 1/5, use 3 × 5 = 15.
Change each fraction: 2/3 = 10/15 and 1/5 = 3/15. Then add: 10/15 + 3/15 = 13/15. If the answer is more than 1, you can write it as a mixed number. Always check whether it can be simplified.
Worked example
3/4 + 2/3 =
- Common denominator 12: 3/4 = 9/12 and 2/3 = 8/12.
- 9/12 + 8/12 = 17/12 = 1 5/12.
Answer:1 5/12
Key words
- lowest common denominator
- the smallest shared denominator
- mixed number
- a whole number and a fraction
- simplify
- divide top and bottom by a common factor
Practise this
New questions every time.
Common Core-referenced: 5.NF.A.1