You can only add fractions directly when the parts are the same size. In 1/2 + 1/4, halves and quarters are different sizes. But 4 is a multiple of 2, so change the halves into quarters: 1/2 = 2/4.
Now add the quarters: 2/4 + 1/4 = 3/4. The rule is: find a common denominator, change the fractions to equivalent fractions with that denominator, then add the numerators.
Worked example
1/3 + 4/9 =
- 9 is a multiple of 3, so change 1/3 into ninths: 3/9.
- 3/9 + 4/9 = 7/9.
Answer:7/9
Key words
- common denominator
- a denominator both fractions can share
- equivalent fraction
- a fraction with the same value
- multiple
- a number in another number's times table
Practise this
New questions every time.
Common Core-referenced: 5.NF.A.1