Adding & subtracting fractions
Adding & subtracting: related denominators
When the denominators are different but one is a multiple of the other, rewrite one fraction so they share a common denominator, then add or subtract.
The method
1. Find a common denominator (a number both denominators divide into). 2. Rewrite the fractions so they both use it. 3. Add or subtract the numerators and keep the common denominator.
Worked examples
Adding
- Work out 2/3 + 1/6.
- 6 is a common denominator. Rewrite 2/3 as 4/6.
- 4/6 + 1/6 = 5/6.
Answer: 2/3 + 1/6 = 5/6
Subtracting
- Work out 3/4 − 1/8.
- 8 is a common denominator. Rewrite 3/4 as 6/8.
- 6/8 − 1/8 = 5/8.
Answer: 3/4 − 1/8 = 5/8
Where you see this in real life
- Combining a half cup and a quarter cup of flour in a recipe.
- Adding parts of a garden used for different vegetables.
- Working out how much of a pizza is left when slices are different sizes.
Key words
- Common denominator
- A number that all the fractions' denominators divide into.
- Equivalent
- Fractions that have the same value, like 2/3 and 4/6.
- Multiple
- The result of multiplying a number by a whole number.