# Two Digit Subtraction With Rounding

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## Objective

Students will be able to use place value and rounding knowledge to create and complete subtraction problems accurately.

#### Big Idea

Putting it all together is an important step that is often missed in math education. This lesson combines place value strategies, subtraction and addition strategies, as well as rounding in the form of a game. What could be better?

## Mini Lesson

10 minutes

The warm up to this lesson is to simply teach the students how to play a game.  For the game, the children draw 4 or 5 cards from a deck of single digit number cards.  Each player must make two numbers to find the difference between. The difference is their points for the round. As they play, they continue to add their differences together until someone reaches, or gets closest to 500.

In this clip, you will hear my students making sense of the procedure of the game. At this point, I don't recommend that you teach students strategy, as the partnerships will end up making that connection on their own, just as this student did during her explanation.

In the second step, we discuss rounding for accuracy before finding the actual difference.  Listen to these two students.  You will hear one still working on the rounding skills, while the other has a strong strategy.

## Active Engagement

30 minutes

Quickly pair the students up and set them to work.  As they work, circulate and probe into why they are creating the numbers they are, and listen to their strategies for subtracting and adding. Remember to use open ended questions, and do not finish students' sentences. If they get stuck, they can draw or use manipulatives to help explain their thinking or you can use simple prompting.

In this video, the student was subtracting from the ten's and then the one's.  It worked for this equation, so I adjusted his cards in the second clip and asked him to solve.  I also gave him a strategy to think about. This "immediate" teaching and learning is invaluable.

In this clip, the student is discussing her strategy for creating numbers.  Aside from working on the appropriate terms for her communication, I also recognize that she needs some assistance in working from the one's to the ten's.  However, I am glad she is working with strategies.

## Sharing and Closing

10 minutes

For our closing, we simply discuss how the game went, what strategies seemed to work well, and how the rounding helped us figure out which numbers to build.

I prefer to stand back, whenever possible, during these "big" discussions. The students did the work, and this is their opportunity to make meaning of their thinking. I'm there to facilitate, and at times I rephrase or echo a student thought or question, to help them "hear" their thinking, or prompt students to show their thinking by making specific reference to a manipulative such as a number line.