# Lesson: Bar Model and Intro. to Adding vs. Subtracting, Day 3

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### Lesson Objective

When using the bar model to solve addition for combination and subtraction for take away problems, mathematicians need to make sure to choose the correct bar model to use.

### Lesson Plan

Edward W. Brooke Charter School

Unit: Add and Subtract within 100

Lesson: 2.3

Teaching Point: When using the bar model to solve addition for combination and subtraction for take away problems, mathematicians need to make sure to choose the correct bar model to use.

Materials:

• 2.3 Problem Solving Task
• Problem Solving Kits

Counting (5 minutes)

Problem Solving Task (5 minutes)

• Read directions and problem solving task to scholars.
• Have scholars read directions / task aloud twice.
• Send students off to work independently.
• NOTE: This problem solving task is 2 pages long!

Teaching (20 minutes)

• Students turn and talk with partners about the bar models that they used to solve each problem and why they used those particular bar models.
• Have a whole class discussion about the two questions.
• Important points to include in the discussion:
• When mathematicians solve a word problem, they need to ask themselves “What is this problem asking me to figure out?”
• You can use the terms “change to more” when talking about addition for combination problems and “change to less” when talking about subtraction for take away problems. Mathematicians ask themselves: “Is this a change to more story or a change to less story?”
• Mathematicians choose a bar model based on whether or not it is a change to more or a change to less number story.
• Do two problems as a whole class (students should do them first on a slate before discussing them). When doing the problems, make sure to have a conversation about why you chose the bar model that you did. Make sure to connect each problem to a number sentence. If necessary, you can take out the linking cubes to decide what type of bar model to use.

Possible problems to use:

There were 9 kids in the pool. 2 more kids got in the pool. How many kids are in the pool now?

There were 9 kids in the pool. 2 kids got out of the pool. How many kids are in the pool now?

Independent Practice (10 minutes)

• Write number stories on the board and have students solve them on their slates using a bar model and a number sentence.
• When students are drawing bar models, make sure that they are drawing them correctly (refer to model bar models in previous lessons.) It is hard to fix bad habits. So, it is better to catch the mistakes early!
• Make sure to vary the wording of the problems so that scholars do not get used to one wording over another.

Math Workout (10 minutes)

Summary

• At the end of this lesson, scholars should understand that mathematicians choose which bar model they will used based on what they number story is asking them to figure out.
• Scholars will have a solid understanding about how to make bar models for addition for combination and subtraction for take away problems.

### Lesson Resources

 2.3 math workout.docx   Activity 528 2.3 problem solving task.docx   Activity 308