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Mental Addition of Two-Digit Numbers

This is a complete lesson with instruction and exercises for the student about adding two 2-digit numbers mentally, meant for 2nd grade. The basic idea is to add in parts, such as tens and ones separately, or add in parts in some other way.



Example 1. Add in parts 40 + 55. 

First break 55 into its tens and ones. 55 is 50 + 5.

So, 40 + 55 becomes 40 + 50 + 5.

Now add 40 and 50. You get 90. Then add the 5. You get 90 + 5 = ______.

Example 2. Add in parts 36 + 30.

First break 36 into three tens and ones. 36 is 30 + 6.

So, 36 + 30 becomes 30 + 6 + 30.

Now add 30 and 30. That is 60. Then add the 6. You get 60 + 6 = _______.

1. Add in parts, breaking the second number into its tens and ones.

a.   20 + 34 = _______

    20 +    3    +    4   

b.  70 + 18 = _______

    70 + ______ + ____

c.  50 + 27 = _______

    50 + ______ + ____

2. Add in parts. Break the number that is not whole tens into its tens and ones in your mind.

a.

17 + 10 = _______

26 + 10 = _______

42 + 10 = _______

b.

16 + 20 = _______

34 + 30 = _______

67 + 20 = _______

c.

50 + 14 = _______

60 + 23 = _______

30 + 45 = _______

3. Add mentally. We already studied these. The first one is the helping problem.

a.

7 + 8 = ______

17 + 8 = ______

37 + 8 = ______

b.

4 + 9 = ______

14 + 9 = ______

44 + 9 = ______

c.

8 + 4 = ______

48 + 4 = ______

78 + 4 = ______

d.

7 + 9 = ______

57 + 9 = ______

37 + 9 = ______



How can you easily add 16 + 19?
Think about it before you go on!

Here's the answer: again, add in parts.
Look at the example on the right.

16 + 19

=  6  +  9  + 10 + 10

=  15  + 10 + 10 = _____

4. Add in parts.

a.      13  +  18

= ____ + ____ + 10 + 10

=

b.      15  +  15

= ____ + ____ + 10 + 10

=

c.      17  +  18

= ____ + ____ + 10 + 10

=

d.      19  +  15

= ____ + ____ + 10 + 10

=

e.   18 + 12 = ______

f.   13 + 16 = ______

g.  16 + 17 = ______

h.   17 + 15 = ______

5. a. Laura owns 13 cats. Five of her cats live in the house.
        How many of her cats live outside?


    b. Laura's cats eat 20 lb of cat food in a week. Laura has two 4-lb
        bags at home. How many more pounds of cat food does she
        need to have enough for one week?



6. Count by threes.

   42, 45, _______, _______, _______, _______,_______, _______, _______

7. Find the pattern and continue it. This pattern “grows” at each step.

+    

+    

+    

+    

+    

+    

+    

+    

1

3

7

13 21 31 _____ _____ _____


Add two-digit numbers: Add the tens and ones separately
Add tens on their own.
Add ones on their own.
Lastly, add the two sums.  

45 + 27

 
40 + 20    +    5 + 7  

60    +  

  12  = 72

8. Add by adding tens and ones separately.

a. 

 36  +  22

 
 30 + 20  +   6 + 2  

______

 + ______

  = _______
b. 

 72  +  18

 
 70 + 10  +   2 + 8  

______

 + ______

  = _______
c. 

 54  +  37

 
 50 + 30  +   4 + 7  

_______ 

+  _______ = _______
d. 

    24   +   55

 
_____ + _____ + ____ + ____  

_______ 

+  _______ = _______
e. 

   36  +  36

 
_____ + _____ + ____ + ____  

_______ 

+  _______ = _______
f.    42  +  68
g.     45  +  18


 
 

h.    37  +  58 

 

   Figure out the missing numbers for these addition problems.

a.
       
+ 1 4
  4 1
b.
     
+   3
  7 1
c.
    
+ 2 5
  5 1
d.
   
+ 7 8
  9 1
e.
    
+ 2 6
  6 1



This lesson is taken from Maria Miller's book Math Mammoth Add & Subtract 2B, and posted at www.HomeschoolMath.net with permission from the author. Copyright © Taina Maria Miller.



Math Mammoth Add & Subtract 2-B

A self-teaching worktext for 2nd grade that covers mental addition and subtraction with two-digit numbers, and regrouping in addition and subtraction (carrying & borrowing).

Download ($5.65). Also available as a printed copy.

=> Learn more and see the free samples!

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