### Lesson plan

# Subtraction Towers

#### Learning Objectives

Students will be able to subtract by finding the unknown addend within a problem.

#### Introduction

*(5 minutes)*

- Present a short scenario to the class. Tell the students that you were asked by a famous toy company to design some towers for a model city.
- Show the class a tower of six interlocking cubes. While holding the tower, tell the class that the current tower is too short and that the toy company said it must have a total of 11 cubes.
- To gain the class's attention, count out 11 cubes and purposely add them to the six cubes. Show the new tower to the class.
- Ask students if they think the toy company would add this tower to their toy model. Ask students to give an explanation on why or why not.

#### Explicit Instruction/Teacher modeling

*(10 minutes)*

- Holding up the tower of linking cubes, explain that the current tower has 17 cubes and therefore will not work for the model. Disassemble to show the original tower of 6 cubes.
- Write
*11 – 6 = _____*on the board. Tell the class that one way to solve this problem is by finding the difference between both numbers, or by using**subtraction**. Explain that the 11 represents the total needed in the tower for it to be used as a model, and 6 are how many we started off with. - Pointing to the equation on the board, ask students if you would be able to show this equation using your current tower. Have students explain why or why not.
- Tell students that addition can be used to find the difference between the two numbers.
- Write
*6 + ____ = 11*on the board. Label and explain that the 6 represents the number of cubes in the tower already and 11 represents the total number of cubes needed. - Circle the empty space on
*6 + ____ = 11*. Label and explain that this area is known as the**unknown addend**. - Tell students that finding the unknown addend can help them solve a related subtraction problem. Show that
*6 + ____ = 11*and*11 - 6 = ____*are related. - Model adding additional cubes to the tower of 6 and stopping at 11. Ask students how many cubes were added to make a total of 11. Tell students that 5 is the unknown addend and write it on the board. Therefore,
*6 + 5 = 11*and*11 – 6 = 5*.

#### Guided Practice

*(10 minutes)*

- Give each student 20 linking cubes.
- Write
*8 + ____ = 15*on the board. Ask students to use their cubes to solve for the unknown addend. Model building a tower of 8 cubes and adding on to reach a total of 15 cubes. Count aloud so your class can hear the unknown addend. - Repeat with
*____ + 5 = 12,**9 + ____ = 12,*and*____ + 3 = 11.* - Next, tell the class that based on their new knowledge, they will solve for
*17 – 9 = ___*with their linking cubes. - Remind students that the subtraction problem can be rewritten as the addition equation,
*9 + ___ = 17*. Allow time for student learning, and then model how to solve. - Repeat using
*13 – 8*and*10 – 4.*

#### Independent working time

*(15 minutes)*

- Have students divide a blank sheet of paper into three equal parts.
- Ask students to copy the following problems at the top of their first column.
*9 + ___ = 12*and*12 - ___ = 9.* - Tell students to create a visual model of the problem in two different colors. For example, the 9 may be drawn as 9 yellow cubes and 3 red cubes may be on top to show a total of 12 cubes.
- Students will solve and provide a visual model for:
*6 + ___ = 13*and*13 - ___ =6*and*___ + 4 = 11*and*11 -4 = ___*in the final two columns. - Monitor the classroom to check for student understanding.

#### Differentiation

**Enrichment:**Students can create their own unknown addend equations for classmates by either providing a written equation or creating a visual model for students, to allow them to write a matching equation.**Support:**Provide small group instruction to assist in subtraction by modeling with linking cubes. Have students practice rewriting subtraction problems as an addition problem, and then solve for the unknown addend.

#### Assessment

*(10 minutes)*

- Visual observations will be made throughout the lesson, as students are working with their linking cubes to solve for subtraction.

#### Review and closing

*(10 minutes)*

- Write a subtraction review equation on the board. Ask students to solve using their linking cubes.
- Have students share how they solved the equation.
- Ask students to share what they learned in today’s math lesson.