This is a complete lesson for 1st or 2nd grade about adding a two-digit number and a multiple of ten, such as 58 + 30. Students first use a visual model for tens and ones to add. Then the lesson explains a strategy for doing such sums: break the 2-digit number into parts before adding. The lesson has many exercises, word problems, a pattern exercise, and a fun puzzle.

1. The numbers are shown with ten-sticks and one-dots. Write the sums.         + a. 54 + 10 = ______              +  b. ______ + 20 = ______       +     c. ______ + ______ = ______  +    d. ______ + ______ = ______    +        e. ______ + ______ = ______   +       f. ______ + ______ = ______

 Adding whole tens and another 2-digit number Break down the other number into tens and ones. Add the tens. Then, add the ones.  2. Add. Break the second number into tens and ones first. Then add the tens.

 a.  10 + 34 = _______      (10 + 30 + 4) b. 10 + 28 = _______      (10 + _____ + _____ ) c.  20 + 24 = _______      (20 + _____ + _____ ) d.  30 + 21 = _______ e.  50 + 17  = _______ f.  40 + 33  = _______ g.  60 + 23  = _______ h.  30 + 37  = _______ i.  70 + 25  = _______

3. Add. Break the first number into tens and ones first. Then add the tens.

 a.  45 + 20 = _______       (40 + 5 + 20) b.  27 + 20 = _______( _____ + _____ + 20 ) c.  45 + 40 = _______( _____ + _____ + 40) d.  46 + 30 = _______ e.  16 + 50 = _______ f.  38 + 60 = _______ g.  20 + 77 = _______ h.  58 + 40 = _______ i.  40 + 39 = _______

4. Explain in your own words how you can mentally add 21 + 60.

5. Fill in the chart of doubles again,
and notice its PATTERN.
 5 + 5 = _______ 10 + 10 = _______ 15 + 15 = _______ 20 + 20 = _______ 25 + 25 = _______
 30 + 30 = _______ 35 + 35 = _______ 40 + 40 = _______ 45 + 45 = _______ 50 + 50 = _______

6. Isabella got 30 books out of the library, and read half of them in two
days. How many books does she have left to read?

7. Gwen and Mom went shopping. They bought shoes for \$40, a top for \$10,
and a skirt for \$20. Mom paid half of the cost and Gwen paid the rest.
How much did Gwen pay?

8. Jacob had \$61. Then he bought a toy for \$30. How much money does he have left?

9. Fill in the missing numbers and find how many tens were added.

 a.  12 + _____ = 22     12 + _____ = 52     12 + _____ = 42 b.  45 + _____ = 65     45 + _____ = 55     45 + _____ = 75 c.   23 + _____ = 63     23 + _____ = 53     23 + _____ = 93

10. Add 10, 20, 30, or 40. In the box below the number, write “E” if the number is even,
and “O”, if the number is odd. What do you notice?

+ 10 12 22 E E

+  20 19 O

+ 30 32

+ 40 37

+ 40 23

+ 30 58

+  20 7

+ 10 85 How many different solutions can you find for this puzzle?   Find at least two. All numbers are whole tens.
 + + = 70 + + = 70 + + + + + + + + = 100 + + = 100 + + + + + + + + = 70 + + = 70 = 80 = 100 = 60 = 80 = 100 = 60

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 standard for 1st grade: 1.NBT.4.

#### 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).