What is subtracting fractions?
Subtracting fractions with different denominators follows the same first step as adding them: find a common denominator, convert both fractions, then subtract just the numerators. The denominator stays the same throughout.
Why it matters
Fraction subtraction is the natural partner to fraction addition and it unlocks a whole category of word problems: how much flour is left, how far apart two measurements are, how much time remains. Kids who have the LCD move solid from addition find subtraction almost automatic.
Worked example
Subtract: 5/6 − 1/4.
- 1
The denominators are different (6 and 4). Find the LCD: the smallest number both 6 and 4 divide into evenly.
Same opening move as adding fractions. You always need the same-size pieces before you can combine or compare them.
- 2
Multiples of 6: 6, 12, 18, 24, ... Multiples of 4: 4, 8, 12, 16, ... LCD = 12.
- 3
Convert 5/6 to twelfths: 6 × 2 = 12, so multiply top and bottom by 2. 5/6 = 10/12.
- 4
Convert 1/4 to twelfths: 4 × 3 = 12, so multiply top and bottom by 3. 1/4 = 3/12.
- 5
Subtract the numerators: 10/12 − 3/12 = 7/12. The denominator stays 12.
You have 10 twelfths and you take away 3 twelfths. That leaves 7 twelfths.
- 6
Check whether 7/12 simplifies. GCF of 7 and 12 is 1, so 7/12 is already in simplest form.
Answer
7/12
Common mistakes
- •Subtracting the denominators too, writing 5/6 − 1/4 = 4/2. The denominator describes the piece size, not a quantity you subtract.
- •Changing the denominator to the LCD but subtracting in the wrong order, computing 3/12 − 10/12 and getting a negative fraction when the problem clearly has a positive answer.
- •When subtracting mixed numbers: forgetting to borrow from the whole-number part when the first fraction is smaller than the second. For example, 3 1/4 − 1 3/4 requires rewriting 3 1/4 as 2 5/4 before subtracting.
- •Applying the LCD to only one fraction instead of both, leaving one denominator unchanged and then subtracting mismatched pieces.
How Briveli teaches subtracting fractions
Briveli covers subtracting fractions in Grade 4 (same denominators) and Grade 5 (unlike denominators and mixed-number subtraction with borrowing), keeping the LCD method consistent with the addition work in the same unit.
Practice Grade 4 math on Briveli