## Lesson: Adding and Subtracting Two-Digit Numbers and Money AmountsIntroducing the Concept

Begin by reviewing basic addition facts. When children demonstrate proficiency with these, explain that they will learn next how to add two-digit numbers. Ask them for examples of things in the classroom that would be represented by two-digit numbers, such as the number of books, desks, and coat hooks. Then ask them to suggest several examples of two-digit numbers and write them on the board.

Materials: blank transparency, overhead base-ten blocks, coins

Preparation: Draw a tens-and-ones frame on the transparency. Have children practice using a tens-and-ones frame and base-ten blocks to represent two-digit numbers.

Prerequisite Skills and Concepts: Children should have a solid grasp of the basic addition facts.

• Say: Today we are going to learn how to add two-digit numbers.
Using the overhead base-ten blocks, show 3 tens and 4 ones.
• Ask: I have 3 tens and 4 ones. What number does this show? (34)
Then show 5 tens and 2 ones.
• Say: I have 5 tens and 2 ones. What number does this show? (52)
Children may respond that you can combine the blocks to see how many tens and ones there are altogether. With the class, count the ones and tens.
• Ask: What is the sum of 34 and 52? (8 tens and 6 ones, or 86)
• Say: We can use a tens-and-ones frame to help us add 34 and 52.
Write a 3 in the tens column of the tens-and-ones frame and 4 in the ones column. Then place the 5 in the tens column and the 2 in the ones column. Explain that when adding, always begin the addition with the ones column.
• Ask: Who can tell me what 4 ones plus 2 ones are? (6 ones)
Write the 6 in the ones column. Who can tell me what 3 tens plus 5 tens are? (8 tens)
• Ask: What is the sum of 34 and 52? (86)
Have children note that the sum is the same as the sum they found using the blocks.
Replace the base-ten blocks with coins representing 34¢ and then 52¢. Have children count the dimes and pennies for each amount.
• Ask: Suppose I want to add 34¢ and 52¢. How can I show addition with the coins?
Children may suggest combining the dimes and then the pennies, and count them to find how many dimes and pennies there are in all. Have children determine that there are 8 dimes and 6 pennies, or 86¢.
• Say: You can add money amounts just as you add numbers. Just remember to write a cents symbol for each amount.

Write 34¢ and 52¢ in the tens-and-ones frame. Relate the dimes to the “tens” and the pennies to the “ones.” Beginning with the pennies, work through the addition. Explain that when adding money amounts less than a dollar, we must write a cents symbol after each amount. Conclude by referring children to the original example and noting that the only difference between adding money amounts and adding counting numbers is the cents symbol (¢).

You may wish to repeat the activity using other examples that do not require regrouping.