1. Strand B- Number Sense Grade Three
Mental Math
Strand B- Number Sense
Grade Three

2. Double Facts
If you know that = 12, then applying the doubles strategy to adding numbers in the 10s, you know = 120.
If you know that = 6, then you know that = 60.

3. Double Facts

4. Double Facts

5. Double Facts

6. Near Doubles This strategy is also called “doubles plus one.”
The strategy is to double the smaller number and add one.
Example: is (6 + 6) +1.
Now, apply this strategy to 10s and 100s.
Example: is ( ) + 10.

7. Near Doubles

8. Near Doubles

9. Doubles Plus Two Strategy A: doubling the smaller plus 2
- Example: is double 4 plus 2 more.
Extension to Grade Three level:
is double 40 plus 20 more.
Strategy B: doubling the number between
-Example: is (5+1) + (7 – 1) = 12
is ( ) + (70 – 10) = 120

10. Doubles Plus Two 40 + 60 60 + 80 90 + 70 30 + 10 20 + 40 50 + 30

11. Doubles Plus 2

12. Plus or Minus 0 You already know 5 + 0 = 5. You can extend that to
You can also extend that to
= 500 or 500 – 0 = 500.
0 + 90
20 + 0

13. Plus or Minus 0
80 – 0
500 – 0
0 + 30
600 – 0
0 + 80
700 – 0
100 – 0

14. Make 10 or 100
This strategy is used with addends of 8 or 9 or 80 or 90.
Example: –- think: (from the 6) = 10 and then = 15
Or: = (9 + 1) + 5 = = 15

15. Make 10 or 100
8 + 5
9 + 2
8 + 6
8 + 3
9 + 5
5 + 8
9 + 4
4 + 8
9 + 6
8 + 2

16. Make 10 or 100

17. Plus 2s or “Next Even/Odd” Number
You know that = 7 because when adding 2, you jump to the next odd number since 5 is an odd number. Another example is: = 8 because adding 2 to an even number means you jump to the next even number which is 8.
This can be applied to 10s and 100s.
Example: = 90
= 60

18. Plus 2s or “Next Even/Odd” Number

19. Plus 2s or “Next Even/Odd” Number

20. Plus 3s or “Adding 2 and Then 1”
When adding 3 to a single digit number, think “next even/odd and then next number.”
Example: is (6 + 2) + 1 = 9
8 + 3
5 + 3
9 + 3
3 + 3
1 + 3
7 + 3
2 + 3
4 + 3

21. Plus 3s

22. Plus 3s

23. Front End Addition
Add the highest place values and then add the sums of the next place values.
Example: think and 5 + 7
or = 62.

24. Front End Addition 15 + 66 74 + 19 33 + 21 + 42 25 + 11 + 43

25. Front End Addition 24 + 31 + 14 34 + 11 + 52 24 + 12 + 33 19 + 42

26. Finding Compatibles
Find pairs of numbers that add to powers of ten to make the addition easier.
Example:
Think: is 10 plus 8 is = 18.
Example:
Think: is 100, so 100 and 40 is 140.

27. Finding Compatibles 6 + 9 + 4 2 + 3 + 8 4 + 6 + 2 1 + 9 + 5 5 + 6 + 5

28. Finding Compatibles 10 + 60 + 90 30 + 50 + 70 80 + 30 + 20

29. Break Up and Bridge think 45 and 30 is 75 + 8 = 83
Start with the first number and add the values in the place values beginning with the larger of the second number. (You may want to use the 100s chart as a tool to teach this strategy.)
Example:
think 45 and 30 is = 83

30. Break Up and Bridge 37 + 45 72 + 28 25 + 76 38 + 43 59 + 15 66 + 27

31. Break Up and Bridge 54 + 33 82 + 24 67 + 32 43 + 31 15 + 59 27 + 72

32. Compensation
Change one number to 10, carrying out the addition and then adjusting the answer to compensate for the original change.
Example:
Think– 52 plus 10 is 62, but I added 1 too many, to take me to the next 10 to compensate, so I subtract one from my answer, 62 to get 61 OR
= ( ) – 1 = 61

33. Compensation
43 + 9
56 + 8
79 + 2
48 + 5
3 + 78
2 + 97

34. Compensation
5 + 84
2 + 97
8 + 65
9 + 34
35 + 3
54 + 7

35. Subtraction – Doubles
Use addition double facts to help find the answers to related subtraction combinations.
Example: 12 – 6
Think – 6 + _____ = 12
120 – 60
Think – 60 + _____ = 120

36. Subtraction – Doubles 60 – 30 100 – 50 20 – 10 40 – 20 120 – 60
180 – 90
140 – 70
160 – 80

37. Subtraction – Doubles 200 – 100 800 – 400 1000 – 500 1200 – 600
1600 – 800
1800 – 900
400 – 200
600 – 300
1400 – 700

38. Subtraction – Near-Doubles
When the part being subtracted is close to half of the total, we can think of an addition double fact, and then adjust it by 1 to find the answer.
Example: 9 -4
Think– = 8, = 9, so 9 – 4 = 5
Example: 90 – 40
Think – = 80, = 90, so 90 – 40 = 50