## HOW MANY COOKIES? HOW MANY CRACKERS? [DAY 2]

### ADDING AND SUBTRACTING WITHIN 100

Introduction to using place value strategies to add and subtract within 100

## Intentionality & Unit Overview

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Students will explore the place value strategy of subtracting in parts from a multiple of ten minuend.

## Intentionality…

The purpose of the Day 2 activities is to reinforce key concepts from Day 1. 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.

• The action of removal is represented by subtraction.
• Subtraction problems within 100 can be solved using place value strategies. When students are using place value strategies to solve addition and subtraction problems within 100, they are using their understanding of tens and ones and decomposition to accurately, flexibly, and  efficiently solve problems. See the Student Approaches from the Day 1 Guide for examples.

Note:

• Minuend: The quantity from which another quantity, the subtrahend, is to be subtracted. In the equation 50 − 16 = 34, the minuend is 50.
• Subtrahend: (the number to be taken from another (which is called the minuend) in the operation of subtraction.

Definitions from Wordnik

## Math Talk

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## While Students Are Practicing…

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## Questions: Subtracting Within 100

### Question #1:

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### Question #2:

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### Question #3:

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### Question #4:

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### Question #5:

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Closest to Zero is a game that practices subtracting using a place value strategy.

While your students play or during a wrap discussion, the central point to talk about is how when we are subtracting 26, we can do that by first subtracting 2 tens and then subtracting 6 ones. Some students may subtract the ones before the tens which is also fine.

Another interesting point to discuss is how students handle a situation such as 72 – 18 or 64 – 19, where the subtrahend has more ones than the minuend. So they break up the ones to stop at a multiple of ten? Do they subtract all of the ones at once?

We suggest collecting this reflection as an additional opportunity to engage in the formative assessment process to inform next steps for individual students as well as how the whole class will proceed.

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