# Add a Two-Digit Number and a Single-Digit Number Mentally

This is a complete lesson for 2nd grade math where children learn to add a 2-digit number and a single-digit number mentally, without crossing to the next ten, such as 35 + 4 or 61 + 5. They add "within" the same ten. The lesson contains both instruction and plenty of exercises, and even a 'bonus' puzzle problem in the end.   + 25 + 3 = 28

The 2 tens do not change.  + 12 + 7 = 19

The ten does not change.    + 34 + 4 = 38

The 3 tens do not change.

1. Write an addition sentence for each picture.

 a.    + _______ + _______ = ______

 b.    + _______ + _______ = ______

 c.     + _______ + _______ = ______

 d.     + _______ + _______ = ______

2. Add. Compare the problems. The top problem helps you solve the bottom one!

 a.  5  +  2  = _____    35  +  2  = _____ b.  4  +  5  = _____    64  +  5  = _____ c.  3  +  6  = _____    93  +  6  = _____

3. Add. For each problem, think about a “helping” problem with numbers less than 10.

 a.  52 + 7 = _____    2   +   7   = _____ b.  33 + 1 = _____ ____ + ____ = _____ c.  11 + 5 = _____ _____ + ____ = _____

4. The numbers are written in boxes! Add ones in their own column.
Copy the tens number down below.

a.  35 + 3

 tens ones 3 5 + ↓ 3 3 8

b.  12 + 6

 tens ones 1 2 + ↓ 6

c.  57 + 1

 tens ones 5 7 + ↓ 1

d.  64 + 3

 tens ones 6 4 + ↓ 3

5. Now you write the numbers in the boxes. Add ones in their own column.

a.  26 + 3

 tens ones + ↓

b.  72 + 4

 tens ones + ↓

c.  65 + 4

 tens ones + ↓

d.  81 + 4

 tens ones + ↓

 a.6 + 2 = ______ 16 + 2 = ______ 36 + 2 = ______ b.4 + 3 = ______ 24 + 3 = ______ 34 + 3 = ______ c. 5 + 4 = ______ 45 + 4 = ______ 65 + 4 = ______ d. 11 + 7 = ______ 61 + 7 = ______ 41 + 7 = ______

 a. 20 + 5 + 2 = ______ 44 + 2 + 2 = ______ b. 93 + 1 + 5 = ______ 83 + 4 + 3 = ______ c. 100 + 5 + 4 = ______ 52 + 4 + 2 = ______

Remember how to break
a number into its
TENS and ONES?   23 = 20 + 3 tens ones     47 = 40 + 7 tens ones

8. Break the numbers into tens and ones or do it the other way around.

 a. 18  =     10   +    8      25  = _____ + _____ 55  = _____ + _____ b. 32  = _____ + _____ 95  = _____ + _____ ______  =  40   +   9 c. _______ = 60  +  6 _______ = 9  +  80 _______ = 8  +  70

9. Compare. Write < > , or = .

 a.  24 + 3 24 + 5 b.  83 + 5 85 + 3
 c. 17 + 2 19 + 2 d.  36 + 4 46 + 4
 e.  58 8 + 51 f.  66 5 + 61  is a number we don't know—a mystery number! Your task
is to compare without knowing the mystery number!
For example, which is more, + 2  or + 7?

Write < or  > in the boxes. Note: t
here is one comparison you cannot do without knowing the mystery number. Can you find it? + 5 + 4 – 5 – 4 – 5  + 2 + 7 – 5 – 6 +  + 20

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. The lesson addresses the following Common Core standards for 2nd grade: 2.OA.1 and 2.NBT.5.

#### 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).