Adding and subtracting fractions
Lesson 6 of 11
Finding a common denominator
Objective. Add fractions with different denominators.
The key idea
Fractions can only be added when the pieces are the same size. Rewrite both fractions over a common denominator, then add the numerators only.
Worked example
Work out 2/3 + 1/4.
- 1A common denominator of 3 and 4 is 12.
- 22/3 = 8/12 and 1/4 = 3/12.
- 38/12 + 3/12 = 11/12.
Answer: 11/12
Practice
Each question comes with a picture and a listen button. Try it first, then reveal the answer.
Common mistake. Adding denominators as well as numerators, giving 3/7 for 2/3 + 1/4.
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