I tell students that today we will go over some of the things we have been working on for the past few weeks. I begin by writing the following on the board:
15 + 23 + 25 + 27 =
I ask students to think of what might help with solving this problem (model with math MP4) (partners of 10, adding tens, adding ones, using a number line or number grid.) I ask them to choose a strategy and solve the problem.
I give students a few minutes to work and then ask for a volunteer to come up and show us how they solved the problem. I ask if anyone solved it differently and if they can show us what they did. We discuss the similarities and differences in the approaches.
I now put up a second problem:
15 + 35 + 32 + 38
I ask students to think about what they just saw their classmates doing as they made sense of the problem and solved it (MP1), and what they did, and to solve this problem. Again we share several different solutions.
I tell students that today we will be working in centers and I ask them to clear their desks and be ready to listen to the directions for each center. I count back from 35 to give students time to clean up.
I explain the 3 centers to the students and tell them that they will rotate through all 3.
Center 1: At this center students will be working on ways to create and solve 2-digit subtraction problems using manipulatives for support (make sense of problems and solve them MP1). Students will review subtraction houses and how to build a larger number and take away from that number to find an answer. The written problem will be posted to allow students to see how the problem looks on paper as well as how it is solved concretely.
Center 2: Students will use their base 10 blocks to arrange 4 digits into different subtraction problems to determine the greatest and least possible answers (model with mathematics MP4). Example, I give the students the digits 4, 5, 6, 7. The possible subtraction problems are 76 - 45, 76 - 54, 75 - 64, 75 - 46, 74 - 65, 74 - 56, 67 - 45, 67 - 54, 56 - 47. Students can solve these to find the greatest and least possible answers.
Center 3: Students will write a subtraction number story for others to solve. They will draw 4 cards to create 2 2 digit numbers. They will then use those numbers to write a number story. They will solve the story and write the answer on the back. They will exchange stories and solve each other's stories.
I bring students to the rug to close today's lesson. I have gathered up the word problems that they wrote. I invite several students to come up to the easel to solve the problem of a classmate. I invite a child to read his/her problem while 2 children work at the easel to solve it. The rest of the group can try to solve the problem in their heads, or watch what the 2 children are doing. If the child at the easel becomes stuck, they can ask for help from a classmate.