This document provides instruction on adding and subtracting fractions with unlike denominators. It explains that a common denominator is needed and demonstrates finding the lowest common multiple of the denominators by listing multiples and using the first number that appears in both lists. Several examples are worked through step-by-step, showing the identification of the common denominator and the calculation. Students are then provided practice problems with answers on the following slide to check their work.

2. Adding & Subtracting Fractions
Teacher Notes:
• Use this presentation to teach the steps
for adding and subtracting fractions with
unlike denominators.
• It is assumed that students have a
thorough understanding of the process for
making equivalent fractions.
• CLICK HERE to view a demonstration with
opportunities for students to practice
making equivalent fractions.

3. We need a common
denominator to add
these fractions.

4. 7, 14, 21, 28, 35…
2, 4, 6, 8, 10, 12, 14, 16, 18, 20
We need a common
denominator to add
these fractions.

5. 7, 14, 21, 28, 35…
2, 4, 6, 8, 10, 12, 14, 16, 18, 20
The first number IN COMMON
that appears on both lists
becomes the common denominator

6. x 2
x 2
X 7
x 7 7
6
7 + 6 = 13
13

7. We need a common
denominator to add
these fractions.
5, 10, 15, 20, 25, 30, 35, 40, 45
7, 14, 21, 28, 35, 42, 49, 56, 63

8. x 7
x 7
X 5
x 5 15
7
15 + 7 = 22
22

9. We need a common
denominator to add
these fractions.
7, 14, 21, 28, 35, 42, 49, 56, 63
8, 16, 24, 32, 40, 48, 56, 64, 72

10. x 7
x 7
x 8
x 8 32
21
32 + 21 = 53
53

11. We need a common
denominator to add
these fractions.
3, 6, 9, 12, 15, 18, 21, 24, 27
5, 10, 15, 20, 25, 30, 35, 40, 45

12. X 3
x 5
x 5
x 3
9
10
9 + 10 = 19
19

13. We need a common
denominator to add
these fractions.
3, 6, 9, 12, 15, 18, 21, 24, 27, 30, 33
11, 22, 33, 44, 55, 66, 77…

14. x 11
x 11
X 3
x 3 15
22
15 + 22 = 37
37

15. We need a common
denominator to add
these fractions.
3, 6, 9, 12, 15, 18, 21, 24, 27
6, 12, 18, 24,…

16. x 1
x 1
X 2
x 2 4
1
4 + 1 = 5
5

17. We need a common
denominator to add
these fractions.
11, 22, 33, 44, 55, 66, 77…
6, 12, 18, 24, 30, 36, 42, 48, 54, 60, 66, 72

18. x 11
x 11
X 6
x 6 18
11
29
18 + 11 = 29

19. We need a common
denominator to add
these fractions.

20. We need a common
denominator to add
these fractions.
9, 18, 27, 36, 45, 54, 63, 72, 81
5, 10, 15, 20, 25, 30, 35, 40, 45

21. 9, 18, 27, 36, 45, 54, 63, 72, 81
5, 10, 15, 20, 25, 30, 35, 40, 45
The first number IN COMMON
that appears on both lists
becomes the common denominator

22. x 5
x 5
x9
x 9
36
35
36 + 35 = 71
71
36 + 35 = 71

23. We need a common
denominator to add
these fractions.
12, 24, 36, 48, 60, 72, 84, 96
8, 16, 24, 32, 40, 48, 56, 64, 72, 79, 80, 88, 96

24. x 3
x 3
x 2
x 2
19
10 + 9 = 19
9
10

25. 5, 10, 15, 20, 25, 30, 35, 40, 45, 50, 55
We need a common
denominator to
subtract a fraction
from another.
11, 22, 33, 44, 55, 66, 77…

26. x 5
x 5
x 11
x 11
4
15 - 11 = 4
15
11

27. We need a common
denominator to
subtract one fraction
from another.
2, 4, 6, 8, 10, 12, 14, 16, 18, 20 …
10, 20, 30…

28. x 1
x 1
x 5
x 5
2
5 – 3 = 2
5
3

29. Try These
A
F
E
B
C
D

30. Answers On Next Slide
• Each click on the next slide
reveals an answer.
• Check your papers.
• If you discover an incorrect
answer, be able to explain
your mistake.

31. Try These
A
F
E
B
C
D
17
27
19
20
10
9
41
28
26
21
13
12