Number, Number Sense and Operations
6. Model, represent and explain subtraction as comparison, take-away and part-to-whole; e.g., solve missing addend problems by counting up or subtracting, such as “I had six baseball cards, my sister gave me more, and I now have ten. How many did she give me?” can be represented as 6 + ? = 10 or 10 - 6 = ?.
12. Demonstrate multiple strategies for adding and subtracting 2- or 3-digit whole numbers, such as:
a. compatible numbers;
b. compensatory numbers;
c. informal use of commutative and associative properties of addition.

Technology

Technology for Productivity Applications
A4. Use a variety of computer and multimedia technology resources for directed learning activities (e.g., computer, VCR/DVD player, audio player, camera).
B2. Identify and use input and output devices to operate and interact with computers and multimedia technology resources (e.g., scanner, digital camera, video camera).

Technology and Communication Applications
A1. Use media to view information.

Begin by going to the Base Blocks web site (in the Links Section below).

Please note that the web site requires Java.  If your computer does not have Java (or your Java is too old) you can download the most recent version (for free) from the Java web site (in the Links Section below).  Or you can contact a tech person to assist you with this.

You should practice at the site ahead of time to get comfortable using this tool.  There are instructions on the site itself (which you can get to by click "Instructions" at the top of the Base Blocks page), or you can read a copy of those instructions in the File Section below.

Explain to the students how numbers can be represented with base blocks.  Do several examples of simply representing numbers as blocks, and determining what number is being represented by blocks.

To create your own problems on the web site, click the "Create Problem" button.

Since you will only be dealing with 2 and 3-digit numbers, feel free to reduce the number of columns visible by clicking the blue arrows next to "Columns".

Note: During each of these steps, feel free to have a student come to the computer to do the activity (after you have shown how).  Use this as an opportunity to talk about how to use the mouse properly.

Step 2: Base Blocks web site - Subtracting without borrowing

Now show how base blocks can be used to subtract numbers.  Begin with small numbers and work up to 3-digit numbers.  At this point do not introduce problems that require borrowing.  To make this more engaging, feel free to make the subtraction problems practical.  For example you could model such situations as:

• 1 digit - You have 8 pieces of candy.  You eat 3.  How many are left?
• 2 digit - We have 25 students in class normally.  However, 12 of them are out sick.  How many students would there be left in class?
• 3 digit - You want to buy a Chihuahua puppy that costs $435.  You have saved $315.  How much more money do you need?

Step 3: Base Blocks web site - Subtracting with borrowing introduction

Now pose a question that requires borrowing, such as:

• You want to buy a Chihuahua puppy that costs $435.  You have saved $178.  How much more money do you need?

Set up the problem with the virtual Base Blocks, but do not solve it.  Simply show the problem we run into when we try to subtract bigger numbers from smaller ones.  Ask the students to think about how we will solve this while they watch a video clip about this.

Step 4: Video Clip - Subtracting with borrowing

Give a quick introduction about the video clip (preview it ahead of time).  Then show the video clip and feel free to pause it as needed to emphasize points or ask questions.  The link to the video clip in below in the links section.

Note: To access the video clip directly you will need your NCCS username and password.  Otherwise the video clip can be downloaded from United Streaming.  Either way, it is recommended that you save the video clip to your computer, rather than try to stream it live from the web site.  To save it, right-click on the link and choose "Save target as..."

For schools that do not have subscriptions to United Streaming, the video can probably be borrowed from your area media center.  The title is "Mathicas Mathshop - Away with Wishes".

Step 5: Base Blocks web site - Subtraction with borrowing

Now return to the Base Blocks web site, model, and complete the earlier borrowing problem.  As time allows, feel free to do model more problems.

Step 6: Assessment

Give the students the assessment handout (in the Files Section below).  This is a short worksheet to review what they covered in the lesson.

Step 7: Evaluation

When the lesson is complete, please fill out the lesson evaluation (in the Files section below) and return it to the tech team. This will help us know the value of this lesson and how we can improve it for the future.