Adding and subtracting fractions · Year 8
In Year 8 you learn: fractions in algebra and problems.
You can only add or subtract fractions when the pieces are the same size, which means the denominators must match. If they don’t, rewrite the fractions as equivalent fractions with a common denominator first.
Key facts
- Different denominators: first change both to a common denominator (the lowest common multiple is easiest). Then add or subtract.
- Never add the bottoms: 1/2 + 1/3 is not 2/5.
- Mixed numbers: add the wholes and the fractions separately, or turn both into improper fractions.
- When subtracting mixed numbers you may need to exchange a whole: 4 1/5 = 3 6/5.
- Simplify the answer at the end.
Worked examples
Example 1
Work out 2/3 + 1/4.
Answer: 11/12
Example 2
Work out 3 1/4 − 1 2/3.
Answer: 1 7/12