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 +    30   +    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 = _____

 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 _____ _____ _____

 45 + 27 40 + 20    + 5 + 7 60    + 12 = 72

 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 © 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).