Adding & Subtracting Fractions (Unlike Denominators)
This topic covers 5 learning steps, guiding your child from the basics through to confident problem-solving. Each step includes a worked example and adaptive practice questions.
What Your Child Will Learn
- Finding Common Denominators
Learn to find the lowest common denominator for two fractions - Add Unlike Fractions
Convert to a common denominator and add - Subtract Unlike Fractions
Convert to a common denominator and subtract - Mixed Operations
Combine addition and subtraction with unlike denominators - Challenge — Multi-Step Problems
Solve complex word problems involving unlike-denominator fractions
Common Mistakes
- Adding fractions by adding the numerators AND the denominators (e.g. 1/3 + 1/4 = 2/7)
You cannot add fractions with different denominators directly. First find a common denominator: 1/3 = 4/12 and 1/4 = 3/12, so 1/3 + 1/4 = 7/12. Use fraction strips to prove it visually. - Choosing any common multiple instead of the lowest common denominator, then struggling with large numbers
The lowest common denominator keeps numbers small and manageable. For 1/4 + 1/6, the LCD is 12 (not 24). List the multiples of each denominator and pick the smallest shared one.
Tips for Parents
- Use fraction strips or a fraction wall to show why you need a common denominator — the pieces must be the same size before you can add them.
- Practise finding common multiples: "What numbers appear in both the 3 times table and the 4 times table?" The smallest one (12) is the lowest common denominator.
- Use food: "You ate 1/3 of the cake and I ate 1/4. Did we eat more or less than half?" Cut a real cake to check.
- Draw two identical rectangles, divide one into thirds and the other into quarters, then find a way to divide both into the same number of parts (twelfths) so you can compare.
Key Words
- Unlike denominators — Fractions with different bottom numbers — like 1/3 and 1/5.
- Common denominator — A shared bottom number that lets you add or subtract fractions — for 1/3 and 1/4, a common denominator is 12.
- Lowest common multiple (LCM) — The smallest number that appears in both times tables — the LCM of 3 and 4 is 12.
- Equivalent fraction — A fraction with different numbers but the same value — 2/6 = 1/3.
Where This Fits
Before this topic: Children should add and subtract fractions with the same denominator and understand equivalent fractions.
After this topic: Adding and subtracting unlike fractions leads to working with mixed numbers, multiplying fractions, and converting between fractions, decimals, and percentages.
How MathCraft Teaches This
In MathCraft, Adding & Subtracting Fractions (Unlike Denominators) is taught through the Number & Fractions adventure track. Your child follows guided lessons with friendly characters, works through examples step by step, then practises with questions that adapt to their level.
The adaptive engine tracks mastery across all 5 steps, revisiting concepts your child finds tricky and advancing when they're ready. Parents can see detailed progress in the Parent Dashboard.
Practise Adding & Subtracting Fractions (Unlike Denominators) with MathCraft
Step-by-step lessons, worked examples, and adaptive practice — all wrapped in an adventure game your child will love.
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