Decomposing and Composing 20
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Students learn to decompose and compose the number 20. They learn to recognize decompositions on the rekenrek but also use arrows for number bonds and numbers. They learn to compose numbers by counting on.
It is useful to know how that you can split 20 into different groups. You can divide 20 notebooks into two piles of different amounts. An example is 13 and 7, but also 18 and 2.
Students practice counting to 20 and write down the number of counted objects.
Show that you can decompose the number 20 in different ways on the rekenrek. If you already have one of the numbers, you simply count on to see how many are needed. Explain that that is called composing a number. Ask students to practice decompositions of 20 with you on their rekenreks. Show all decompositions of 20. Check that students have understood by having them write down their own decompositions. Next, explain that it is sometimes possible to see the decomposition in an image by color and that the numbers in a number bond can be switched. Discuss that you can also use an arrow to show the number bonds. Practice recognizing the numbers in the images with beads and discuss with students which numbers they can switch, and where number bonds and arrows could be used. Explain that when a decomposition isn't visible by color, that you count on to discover the missing number. Practice this together and check that students are able to count on. There are also decompositions that only have a number. Ask the students to decompose in their heads, but they can use blocks to support their thinking.
Check that students understand decomposing and composing 20 by asking the following questions:
- What does it mean to decompose a number?
- How can you check if your number bond is correct? (count on)
- Can you name a few decompositions of 20?
Students practice composing 20 by counting on to find the missing number in the number bond, they are supported by objects. Next they will have to find the missing number without the support of objects.
Check that students can decompose 20 by having them fill in the missing number in a decomposition. Discuss that it is useful to be able to decompose and compose 20 so you can divide objects in different ways. You can choose to draw more arrows for number bonds and ask students to complete them for 20.
Students who are challenged by decomposing can be supported by the use of manipulatives. They can also use a rekenrek to help visualize the decompositions.
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