In this lesson, students will learn how to mentally add multiples of ten to a two-digit number by modeling using tens rods and unit cubes.
Objective: Develop number sense for whole numbers to at least 1,000: Use 10’s and 100’s as units for counting, increasing, and decreasing quantities by 10’s and 100’s from any given number.
• Place-value blocks(a.k.a. base ten blocks) (9 tens rods and 9 unit cubes per student) - Purchase here, download this sheet to make your own here, or you can use Legos (one block = 1 one, stack ten blocks together to = 1 ten)
1. Begin the lesson by asking students if they have ever seen an adult adding numbers quickly in their head. Ask them to think about where the adult was and what they were doing. Then, call on a few students to share their stories. Talk about how they have been working on memorizing our addition facts so they can quickly add mentally. Then, introduce today’s lesson by telling students they are going to learn how to add mentally faster by learning how add tens to two-digit numbers.
2. Now, explain that in order to do this they will have to know how to count by 10s! Ask students if you are going to count by 10s, what number will you begin with? After someone tells you “10”, ask the class to help you count by 10s. Do this until you reach 100.
3. After quickly reviewing how to count by 10s, write a problem on the board such as 44+20=___. Ask students what strategies they can use to add those two numbers. Give them a minute or two to think about it and use the strategy they come up with to solve the problem. After students have a few minutes to work on the problem, ask volunteers to share with you how they found their answers. As they share with you, model what they are telling you on the board. Expect students to tell you they can count by tens to add mentally or add tens first and then add ones.
4. Next, have students use their tens rods and unit cubes to model the addition problem on the board. This will help students remember how to use the place value blocks, and they will also be able to see how they can also use these manipulatives to help them add tens to two-digit numbers! Guide students through using the place value blocks to solve the problem. Ask how many tens are in 44? How many ones are in 44? As students tell you the answer, draw four tens rods and four unit cubes on the board for them to see. Now, tell them they want to add 20. How many tens are in 20? Tell them they can count on by tens to find the sum. In another color of marker, draw another tens rod next to the four already there. How much do we have now? Then, draw another tens rod. How many are there now? Now, have the students count aloud as you point to the tens rods after 44. Then write the sum of the problem on the board.
5. Now, it’s time for students to practice some on their own! Have practice problems that require students to add 10, 20, 30, and 40. Give students a problem for each one and guide them as they need it. It would be great practice to pull out a dry erase board and have the class answer as a whole and hold up their boards for you to check. It's a quick and easy assessment to see who gets it and who doesn't.
6. After you have had a chance to watch students practice a few problems, it will be time to let them work on their own! Give them practice problems to do on their own. Walk around the room to make sure everyone understands and help any students who need extra assistance.
Differentiation: When students work on their own, they will have the option as to whether or not to use the place value blocks. For more advanced students who may finish early, you may put extra challenging problems on the board for them to solve. For example, 31+20+40=___.
Assessment: To assess, I will observe students answering problems independently. Be looking to see if students understand that when adding tens to a two-digit number, only the tens digit in the sum changes – the ones digit stays the same.
Remediation/Next Steps: If students struggle with adding tens to a two-digit number, it may be necessary to go back and make sure they understand place value. Review that ten ones equal a tens rod. If students struggle with adding tens, this is often the problem. Have them count out ten unit cubes and explain they can exchange the ten unit cubes for one ten rod. Then, have them model a number such as twelve and ask them to add ten to it. They will use the place value blocks to model the numbers; then, they will count the blocks to find the sum.
For students who are ready to move on to the next level, they will begin to learn how to mentally add a two-digit number and one-digit number. For example, 32+9=__.