Adding and Subtracting Fractions
Solve problems involving addition and subtraction of fractions with the same or related denominators, using different strategies.
Read this first
You can only add pieces that are the same size. Make the bottoms match first.
Both bars are cut into eighths. Now they can be added.
Words you need
- Denominator
- The bottom number. It says how big each piece is.
- Numerator
- The top number. It counts the pieces.
- Related
- One bottom number divides into the other.
Worked example
e.g. What is 1/2 + 1/8?
- The bottoms are 2 and 8. They are different.
- 2 goes into 8. So change halves into eighths.
- 1/2 is the same as 4/8.
- Now add. 4/8 + 1/8 = 5/8.
The bottom stays the same when you add. Only the top changes.
Practice
The pictures fade as you go. By the last few you are on your own.
1What is 2/6 + 1/6?with a picture
Answer3/6, or a half.
2What is 1/4 + 1/8?with a picture
Answer3/8.
3What is 3/4 - 1/4?with a hint
Need a hint?
The bottoms already match.
Answer2/4, or a half.
4What is 1/3 + 1/6?with a hint
Need a hint?
Change thirds into sixths first.
Answer3/6, or a half.
5What is 5/10 + 2/10?on your own
Answer7/10.
6What is 1/2 + 1/4?on your own
Answer3/4.
7Why is 1/4 + 1/4 not 2/8?on your own
AnswerThe bottom stays 4. The answer is 2/4.
Think about it
1
Why must the bottoms match?
One good answerYou cannot add pieces of different sizes.
2
What happens to the bottom when you add?
One good answerNothing. It stays the same.
Common mistakes
Adding the bottom numbers too.
Only the tops get added.
Forgetting to change one fraction.
Both must be over the same bottom first.