# Delightful Decimals

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## Objective

SWBAT apply mental math strategies and properties of addition to decimal addition. SWBAT compare strategies in order to better distinguish the properties of addition.

#### Big Idea

Students begin to use mental math to gain better decimal number sense.

## Intro & Rationale

The purpose of comparing multiple strategies is to use the student's invented method that is comprehensible to him/her in order to make sense of more sophisticated strategies. It helps students see connections between a strategy that makes sense to them and one that is new to them. It also exposes the mathematical foundation. Students then have the option or are directed to try out the new strategy.

## Warmup

15 minutes

On the white board is modeled the strategies 005.JPG that students are comparing in their warmup. There are four strategies modeled by four pretend students (Jenna, Adam, Cesar, and Yolanda). I tell students these are strategies that they have been using and seeing in class during our number talks ("Let's Talk Addition" and "What Were They Thinking?!"  in previous lessons in this unit). I ask them to use the sentence frames for Comparing mental math strategies projected on the screen to help them look for multiple similarities and differences in the strategies. While they are working I periodically ask for a show of hands who has come up with more than one similarity so that they don't stop at just one.

I ask them to share their comparisons with their math family group and write down any additional comparisons they came up with as a result. Then I ask them to share with the class. I follow up with several questions including: "who else noticed that?",  or "where do they see that?", or "does anyone have another way of saying that?", "why do you think he/she chose to do that?", "how does that help us solve it in our heads?".

Ask them where the friendly numbers are in each of the strategies. In Adam's, Jenna's, and Cesar's it is 30, in Yolanda's they may see 20, 40, 60, 10. Then ask if "both" of the original numbers have to be made "friendly". You really want them to see that they only need to make one of the numbers friendly to make it easier to add mentally, so they can apply this to the decimal addition.

## Number Talk with decimals

35 minutes

Number Talks were introduced to the class in a lesson earlier in this unit (Let's Talk Addition).  The problem string for this number talk is:

4.9 + 4.1

6.8 + 3.2

5.7 + 2.3

6.8 + 3.3

3.7 + 5.5