## Task Teacher Guide

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### In This Purposeful Practice…

Students will explore modelling and solving teen number subtraction prompts to lead to the understanding of the more sophisticated subtraction strategy “Down and Under Ten”. Number talks will be consolidated using a number line as a tool to best show why it is easier to get to ten first. The purposeful practice will allow students the opportunity to work with this tool as well.

### Intentionality…

The purpose of the Day 4 activities is to reinforce key concepts from Day 3. Students will engage in a string of related problems through a math talk and will have an opportunity to complete independent purposeful practice. The math talk and purposeful practice serve to develop a deeper understanding of the following big ideas:

- Understanding hierarchical inclusion allows for flexible composing and decomposing of numbers
- Numbers can be decomposed by separating a whole into two or more parts
- Subtraction names the missing part in terms of the whole
- Different subtraction situations will elicit different strategies
- Number relationships provide the foundation for strategies to help students remember basic facts
- Addition and subtraction are related in that addition names the parts in the whole and subtraction uses the whole to find a missing part.
- Subtraction can be used in either take away, comparison, or missing addend situations.
- Models can be used to connect concrete to abstract
- Down and Under Ten is a strategy that is dependent on the knowledge of other facts, known as a derived fact strategy

## Math Talk

### String of Related Problems

In today’s math talk, students will be exposed to a variety of questions that involve subtracting numbers to make the connection between subtracting from a teen number to make ten and the Down and Under Ten strategy. The consolidation of the math talk videos will allow students to see how the use of a ten frame or a number line can assist in explaining their thinking.

Prompts:

**14 – 4**

**14 – 5**

**17 – 7**

**17 – 8**

**16 – 6**

**16 – 9**

### Visual Math Talk Prompt #1

Show students the following visual math talk prompt and be prepared to pause the video where indicated:

The first part of this math talk draws on the relationship in teen numbers that has been discussed the whole unit. On Day two, the math talks subtracted 10 from a teen number, today’s math talks subtract the “ones” from the teen number to make a difference of 10.

A concrete number line is used in the consolidation of this prompt as a visual representation.

With this type of number line, the amount of spaces can be directly counted to confirm the 4 that have been removed.

Discuss with students that we can demonstrate this removal of 4 in just one jump on the number line without having to count back by ones because once we remove 4 from 14, there is 0 left in the ones/singles column.

The second part of this math talk, introduces students to the Down and Under Ten strategy. The idea of this “number string” is for students to use their knowledge of 14-4 in the first portion of the prompt and realize that one more is being removed.

Again the number talk has been used to as a visual tool to make the connection to the first part of the prompt.

**We already know that 14-4 is 10, so now we are just subtracting one more.**

### Visual Math Talk Prompt #2

Show students the following visual math talk prompt and be prepared to pause the video where indicated:

An open number line is used in the consolidation of this prompt. The freedom of the open number line allows students to create a jump that they feel has a value of 7. They do not need to get caught up in counting the amount of spaces.

In saying that, the open number line eliminates the opportunity for students to be able to directly count the spaces between 17 and 10.

This jump in sophistication may be difficult for some students. Consider having the concrete number line still available for students if needed.

Again, the numbers in this prompt are specifically chosen to continue from the first portion of this number string.

**“If we already know 17-7, how can we solve 17-8?”**

### Visual Math Talk Prompt #3

Show students the following visual math talk prompt and be prepared to pause the video where indicated:

The last number string still encourages students to use the information that they gathered to help them.

**“If I know 16-6 is 10, how does that help me with 16-9?”**

This is an example of using the Down and Under Ten strategy. If nine is decomposed into 6 then there is 3 more to subtract. Students may “just know” the answer is 7 because of their understanding with facts of 10.

## Purposeful Practice

### While Students Are Practicing…

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### Game: Show me on a Numberline

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### Small Group Assessment Opportunity

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## Explore The Entire Unit of Study

This Make Math Moments Task was designed to spark curiosity for a multi-day unit of study with built in purposeful practice, and extensions to elicit and emerge mathematical models and strategies.

Click the links at the top of this task to head to the other related lessons created for this unit of study.

### Visual Number Talk Prompt #1

### Visual Number Talk Prompt #2

### Visual Number Talk Prompt #3

### Purposeful Practice Stations and Small Group Instruction