Regrouping with Base-Ten Blocks

Introduction

• Write the problem 15 ________ 5 = ________ on the whiteboard. Represent the missing operation and difference with a blank circle or line. Say, "I have a problem and I need help solving it! I made 15 cookies to give to my friends. My dad ate 5 cookies before he knew I was giving the cookies to my friends! I have 10 friends and I really wanted to give each of my friends a cookie. Will I have enough cookies for my friends? How can I solve this problem?"
• Allow a student or two to share out their ideas about how to solve the problem. Guide them as they share out, and discuss whether the problem needs an addition sign or a subtraction sign. Encourage them to explain their reasoning by using some of the following prompts:
  • What do you mean by...?
  • Can you tell me more about...?
  • Can you give me an example of...?
• Clarify that the subtraction sign is the correct choice because when we subtract, we figure out the difference of two numbers. Explain that the difference is the result of subtracting one number from another. The difference is how much two numbers differ from each other in value. Have students repeat after you, "When we subtract, we find the difference." Come up with a movement to support student understanding.
• Record the subtraction sign in the problem so it reads: 15 – 5 = ________. Next, put students into groups of four and provide whiteboards and whiteboard markers, base-ten blocks, number lines, a hundreds chart, and manipulatives (e.g. beads, gems, stones, etc.) to each group of students.
• Tape the large anchor chart labeled "Subtraction Strategies" on the board.
• Discuss the subtraction strategies aloud with the students. For example, say, "One way you can solve this subtraction problem is to draw a picture." Refer to the visuals on the anchor chart to support student understanding.
• Give students 2–3 minutes to solve the subtraction problem as a group. Have students share the strategy they used out loud. Put a tally mark to the left of the anchor chart on the whiteboard to represent the strategy each group used.
• Explain to the students that each group used a strategy, or plan of action, to solve the problem. Explain to the students that there are many different strategies we can use to solve subtraction problems. Say, "Today, we will learn about what it means to regroup and how regrouping with base-ten blocks can support us when we solve two-digit subtraction problems."