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Young students do not always understand that the strategies they use with smaller numbers work with bigger numbers. The numbers chosen in this activity are designed to encourage students to decompose the subtrahend in order to subtract in friendly parts.
The teacher and the students will read the learning targets together (Slide 2).
The teacher will gather the students together in a central location where the students can view a whiteboard, interactive board, or chart paper. The teacher will pose the following problems one at a time: 16 - 7 =? 56 - 7 =? and 116 - 7=?. The teacher and the students will follow the steps below: The teacher will pose a problem. The students will solve the problem in their head or mentally. When the students have an answer, they will place a thumbs up against their chests. The teacher will call on three students to share their answers. The teacher will record the students’ answers on the board. The teacher will call on three students to share how they solved the problem. The teacher will record their strategy on the board (Slide 3).
The teacher presents 16 - 7 = ? to the class. The teacher is ultimately looking for students to decompose 7 into 6 and 1.
16 - (6+1) = ? or 16 - 6 - 1 = ?
16 - 6 = 10
10 - 1 = 9
The teacher should model the students' strategies on a number line. The teacher may record students’ strategies in the white area on the slide, on chart paper, or on notebook paper under a document camera. Use the checklist/notes to record the strategies used (Slide 4).
The teacher presents 56 - 7 = ? to the class.The teacher is ultimately looking for students to decompose 7 into 6 and 1.
56 - (6+1) = ? or 56 - 6 - 1 =?
56 - 6 = 50
50 - 1 = 49
The teacher should model the students' strategies on a number line. The teacher may record students’ strategies in the white area on the slide, on chart paper, or on notebook paper under a document camera. Use the checklist/notes to record the strategies used (Slide 5).
The teacher presents 116 - 7 = ? to the class.The teacher is ultimately looking for students to decompose 7 into 6 and 1.
116 - (6+1) = ? or 116 - 6 - 1 = ?
116 - 6 = 110
110 - 1 = 109
The teacher should model the students' strategies on a number line. The teacher may record students’ strategies in the white area on the slide, on chart paper, or on notebook paper under a document camera. Use the checklist/notes to record the strategies used (Slide 6).
The teacher will review the strategies used with all three problems using the guiding questions on the slide. If no student decomposed 7 into 6 and 1. The teacher will need to ask, “How could decomposing 7 into 6 and 1 make 7 easier to subtract in this problem?” (Slide 7).
Assessment Strategies:
The teacher will use the checklist/notes to record the strategies used.
ALCOS 2.12). Add and subtract within 1000 using concrete models or drawings and strategies based on place value, properties of operations, and/or the relationship between addition and subtraction; relate the strategy to a written method.
a. Explain that in adding or subtracting three-digit numbers, one adds or subtracts hundreds and hundreds, tens and tens, ones and ones; and sometimes it is necessary to compose or decompose tens or hundreds.
Keywords and Search Tags:
decomposing numbers, subtracting in parts, subtracting within 100