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Students learn to group numbers to add to 20 using three addends. They learn to make ten, and then to add the third addend.
It is important that students learn grouping to add three addends to 20, so that they learn how to add three addends.
Students see all kinds of numbers on the interactive whiteboard. They must select two numbers whose total is 10. They can circle these numbers.
Show students the image. Show that Jose has 3 marbles, Brandon has 7 marbles, and Lynn has 2 marbles. Ask how they can figure out how many marbles the kids have together. Explain that the first step is to see if you can make ten, which two numbers add together to make ten? Then you add the last number. Then show the tennis-ball problem. Search for the two numbers which add to 10 (the first two), and then add the third number. Show this with the next problem, but it is the last two numbers which add to 10 together, so you add those first, and then the third number. Use this to point out to students that it doesn't have to be the first two numbers of the three which add to 10.
Next students practice finding the two numbers that total 10. The students calculate the answers to the problems shown on the board. In the left row, the first two numbers make ten, whereas in the right row, they are mixed up. To finish up, students practice adding the problems presented as story problems.
Check that students understanding adding to 20 making ten with three addends by asking the following questions:
- How do you group numbers?
- What do you pay attention to when choosing addends?
- What do you pay attention to when adding?
- What are the steps you take?
- Do you only pay attention to the first two numbers?
Students start with problems in which the first two numbers make 10. In the second exercise, the first and last number make 10. Finally students use the numbers from a story problem.
Check that students first look to group the numbers to make 10, before adding the final addend. Emphasize that grouping to make 10 is useful to make adding easier. Check that students have understood by asking how they can quickly add three numbers to 20 with three addends. Next, ask students to practice the problems on the interactive whiteboard. End with a game. Set the students in pairs, and give each pair two dice. The first student rolls one number, and then the second student rolls a number. They decide which number they should add to one of the two numbers to make ten, and then they have an addition problem with three addends. They can also be asked to write these numbers down to track their addition.
Students who have difficulty with the order of addition of three addends can first practice searching for the two addends that make 10.
They can use MAB-blocks or other manipulatives to set out each number. They can also practice decomposing 10.
dice and blocks
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