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Year 7 · Maths · Adding and subtracting fractions · Lesson 2

Adding and subtracting fractions: different denominators

Add and subtract fractions with different denominators using equivalent fractions and the lowest common multiple, and compare fractions.

Whiteboard video lesson: Adding and subtracting fractions: different denominators

33-minute video lesson

Teaching, worked examples and 20 practice questions solved step by step.

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What your child will learn

  • Find equivalent fractions
  • Find a common denominator
  • Add fractions with different denominators
  • Subtract fractions with different denominators
  • Compare fractions

The method

  1. Find a common denominator (the LCM).
  2. Change each fraction to that denominator.
  3. Add or subtract the numerators.
  4. Simplify.

Key words: common denominator, equivalent fraction, LCM, add, subtract.

Worked examples

The first three practice questions from the lesson, with every step.

Question 1

Work out 1/2 + 1/8.

  1. Step 1: Common denominator 8: 1/2 = 4/8
  2. Step 2: 4/8 + 1/8 = 5/8
  3. Step 3: Can 5/8 simplify? No.

Answer: 5/8

Question 2

Work out 3/4 − 1/2.

  1. Step 1: Common denominator 4: 1/2 = 2/4
  2. Step 2: 3/4 − 2/4 = 1/4
  3. Step 3: Can 1/4 simplify? No.

Answer: 1/4

Question 3

Work out 1/3 + 5/9.

  1. Step 1: Common denominator 9: 1/3 = 3/9
  2. Step 2: 3/9 + 5/9 = 8/9
  3. Step 3: Can 8/9 simplify? No.

Answer: 8/9