Adding Tens
Objective
Big Idea
Activating Strategy
I like to start this lesson with a counting by tens video:
Ahead of time, I make two sets of cards with numbers 1 through 9 (I like to use index cards) and mix the cards. In the opening of the lesson, I hold up one card from each stack to represent a two-digit number and have students use whiteboards and markers to draw tens and ones to represent the number.
For example: I hold up the number 35 – students will draw 3 tens and 5 ones on their whiteboards. I continue until I have used up all the cards in both decks.
For the standard NBT.C4, as children begin adding tens, some may still need to use cubes or base-ten blocks to show the tens before drawing quick pictures. If children have difficulty making a transition to quick pictures, then they can trace the objects to make their drawings.
Some children may be able to add tens in their heads using addition strategies. These strategies would most likely extend the basic facts to adding tens. A child with strong mental math ability is able to flexibly compose numbers for use in different situations.
I want to encourage children to develop their own strategies for adding tens. I have children describe their strategies (MP2), even error-filled ones, without being interrupted or corrected. Children often discover their own mistakes if allowed to think through their idea as they present it, resulting in a deeper understanding for all.
Teaching Strategies
To access prior knowledge, I ask the students to solve 1 + 7? (also available as a PPT Adding Tens.ppt)
- How did you find the sum? (I counted on 1 from 7 and got 8)
Explain, that just as 1 + 7 = 8, if we have 1 ten and 7 tens, we will have 8 tens. Ask the students:
- What would the sum be if I had 1 ten and 7 tens? (1 + 7 is 8, so 1 ten + 7 tens would be 8 tens, it would be 80)
I like to write the problems on chart paper/board and gave students use base ten blocks or connecting cubes to model how to solve them:
If Jane had 30 pennies and she got 20 more pennies, how many pennies does she have?
After modeling how to solve, have students draw a quick picture model to represent the addition problem. Guide the discussion:
- How did you model the problem? (I put out 2 tens and 3 tens. Then I put the tens together and counted them to find 5 tens or 50.)
- How does your drawing help you solve the problem? (I drew 2 lines for 2 tens, and 3 lines for 3 tens. Altogether there are 5 tens, or 50.)
Some children may draw ones instead of tens. The concept of 1 ten being the same as 10 ones can be a difficult concept for children to grasp. In these situations, I encourage them to trade 10 ones for 1 ten when they can.
I read/write another problem for the class. I have children solve the problem and record their answers on their whiteboards:
Jane has 40 pennies. Kyle has 50 pennies. How many pennies do they have?
I guide the discussion:
- How can you solve this problem without models? (I can think 4 tens + 5 tens = 9 tens; 9 tens is the same as 90. I can start at 50 and count forward by tens to add 4 more tens.)
We work through the following model together: 30 + 40 = _____ _______ tens
- How can you write 30 + 40 using only tens? (I can draw 3 tens and 4 tens.)
- What can you draw for a quick picture of 30 + 40? (I can draw 3 lines for 3 tens and 4 lines to show 4 tens.)
- How can you find the sum of 30 + 40? (I can count all of the tens to find the total of 7 tens, or 70.)
- How can you use your picture to help find the sum of 30 + 40? (I can count the number of lines to find the number of tens.)
We then work through the following model together: 20 + 40 = _____ _______ tens
- Is the sum 6 or 60? (The sum is 60. There are 6 tens in 60, and 6 is only 6 ones.)
In this picture, a student is drawing tens to solve the problem:
Independent Practice
I assign the Adding Tens_worksheet found in the resources for this lesson.
One common error for this standard is that students may write the sums as ones. To help them, I have children use their quick pictures to explain that each line represents a group of 10, and encourage them to count by tens with their pictures.
For struggling students, I work with them in a small group and use numbered cards 10, 20, 30, 40, 50 and base-ten blocks. I show students two cards and have them use the base ten blocks to model the numbers and draw a quick picture. For example:
- I show them the 20 and 10 card. Have students use base ten blocks to build the numbers. Ask, “How many tens are there in all?” (3 tens) Have children write 2 tens + 1 ten = 30 tens. Together count by tens to show that 3 tens is 30. Continue with other cards.
I like to use this strategy because it will help the students build the connection between the concrete counting of tens using base ten blocks, to adding tens.
In this picture, the student is drawing a model to help solve the problems.
Closing/Summarizing
To close out this lesson I put the numbers 10, 20, 30, 40 on index cards. I give each student an index card and use the inside/outside circle to review. I then split students into two groups, and I have the first group sit in a circle facing out and the second group in a circle on the outside of the first group. The students practice adding their cards together. On my signal, the outside circle rotates one student and the activity is repeated.
