Reteaching
Name
■
Using Mental Math to Add
2-3
You can break apart numbers to make them easier to add mentally.
Add 31 + 45 by breaking apart
numbers.
Add 26 + 17 by breaking apart
numbers to make a ten.
Break the numbers into tens
and ones.
Use a number that adds with
the 6 in 26 to make a 10.
Since 6 + 4 = 10, use 4.
tens
31 =
30
ones
Think: 17 = 4 + 13.
+ 1
Add 26 + 4 = 30.
45 = 40 + 5
Add 30 + 13 = 43.
Add the tens: 30 + 40 = 70.
So, 26 + 17 = 43.
Add the ones: 1 + 5 = 6.
Add the sums: 70 + 6 = 76.
So, 31 + 45 = 76.
Find each sum using mental math.
1. 24 + 71 =
2. 36 + 43 =
3. 54 + 23 =
4. 25 + 49 =
5. 37 + 56 =
6. 77 + 13 =
7. Communicate To add 32 + 56, Juanita first added 30 + 50.
What two steps does she still need to do to find the sum?
What is Juanita's sum?
8. Reason How can Steve add 48 + 34 by making a ten?
What is the sum?
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Practice
Name
2-3
Using Mental Math to Add
Use breaking apart to add mentally.
3. 47 + 41
2. 42 + 29
1. 53 + 34
29 = 20 +
34 = 30 +
53 +
= 83
83 +
= 87
= 62
42 +
So, 53 + 34 =
41 =
+ 1
47+
= 87
+ 9 = 71
+ 1 = 88
So, 42 + 29 =
So, 47 + 41 =
Make a ten to add mentally.
4. 27 + 24
24 = 3 +
27 +
6. 38 + 27
5. 54 + 19
19=
= 30
+ 21 = 51
60 +
So, 27 + 24 =
+6
27 =
+ 25
+ 6 = 60
38 +
= 40
40 +
= 65
= 73
So, 54 + 19 =
So, 38 + 27 =
Find each sum using mental math.
7. 52 + 26
11. 43 + 38
8. 47+8
9. 32 + 17
10. 28 + 31
12. 72 + 7
13. 42 + 33
14. 36 + 14
15. Generalize Ashton broke apart a number into 30 + 7.
What number did he start with?
16. What is the sum of 27 + 42 using mental math?
A 68
B 69
C 78
D 79
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