From Tens to Nines

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SWBAT add or subtract nine from a number less than 50 by first adding or subtracting ten and then changing the answer by one.

Big Idea

Fluency in adding and subtracting is expected during second grade. New strategies to add and subtract efficiently provide students with a variety of options to increase fluency.

Warm Up

15 minutes

I begin each day by trying to connect to prior knowledge. This helps by building new information upon already acquired knowledge and/or skills.

I begin with a few quick doubles and partners of ten problems on the board. I ask students to complete these problems in their math journals. I ask for volunteers to explain how they found the answers to the problems. The other students listen to their peer's explanation, look at their own work in their math journals, and correct their own work as needed By watching what one student did, the other students gain new ideas that may help them in the future. 

Next, I pose a few mental math problems adding 10 to a single digit number. This is a form of skip counting by 10 as we can continue to add 10 to each successive answer. (NBT A2) I use student responses to help me assess what information students have retained from the previous lesson. 

I tell students that now that we have warmed up their math brains, we are ready to work in small groups. I tell them that today we will be visiting 4 centers. They may find different things to do at each center so they should look for their name next to the directions for each center. The reason for this is to provide appropriate experiences for all students. I set up opportunities that are appropriate for each learner. I don't want students to be overwhelmed, or bored because there is only one set of options. We review what good center behavior would look like. We talk about staying on task, following directions, being respectful of others at the center, helping others but not giving them the answers and only seeking adult help if no one at the center can figure out what to do. 

I give only a brief explanation of each center before dividing students up. (I make sure that students who will be doing similar work are in the same group so they can help one another.)

1. Subtracting ten and nine from a number and recording the number sentences. One group will use larger numbers in the double digits and hundreds, one group will use numbers to 20 and use base 10 blocks for support.

2. Building a number and then adding 10 more. Using base ten blocks the first student will build a 2 digit number and say what it is. The second student must add a ten and tell what the new number is. The partner numbers will be recorded so that different numbers are used each time.

3. Word problem pages adding and subtracting 10 and 9 from numbers, one within 20, the other within 100

4. Adding and subtracting 10 and 9 on the hundred's chart - coloring patterns.

Lesson Centers

40 minutes

The centers were briefly introduced during the Warm Ups of the lesson. I divide up the students into 4 homogeneous  groups. (The reason for homogeneous groups is because I want to be able to provide the groups with work appropriate for the differing understandings.)

The students spend 10 minutes at each center and then I ring a bell and ask students to move to their next center.

After students have visited all 4 centers I have them return to their desks. I ask them to tell me somethings they know about adding 9 or 10 to a number, or subtracting 9 or 10 from a number. I make a chart of some strategies students suggest. The chart will be posted in the room for students to reference in future lessons.