#### Material

A board divided like the Addition Strip Board in size and number of squares, however:

The horizontal numbers are 1 to 9 in blue, and 10 to 18 in red.

A blue vertical line after the 9 divides the board.

A set of red and blue strips from 1 to 9, like those in the Addition Strip Board.

A set of varnished strips from 1 to 17 squares in length, unmarked.

Pages for combinations, either loose or in 18-sheet booklets.

Booklet pages may show essential combinations such as:

18 – 9 = 17 – 9 = 16 – 9 = | 17 – 8 = 16 – 8 = | 16 – 7 = |

#### Presentation

##### Introduction

- Invite the child to the lesson.
- Show him the subtraction strip board.
- Point out the box of varnished strips
- Ask him to bring the board to the table.
- Bring the red and blue strips, underlay, and box of varnished strips.
- Ask the child to place the blue strips and how there are no numbers on them.
- Briefly talk about the varnished strips and how there are no numbers on them.
- Show the child where to place the varnished strips, longest first, at the top right of the table.
- Place the second longest strip underneath the previous strip; strips aligned to the right.
- Ask the child to place the rest of the varnishes strips.
- “We’ve seen a board very similar to this one, remember? But this one has 1 to 9 in blue with a blue line here in the middle, and 10 to 18 in red.”
- “We can use these strips to cover up the numbers.”
- Place a varnished strip on the red numbers, aligned right, emphasize the right alignment.
- “What number is left?”
- Point to the number that’s immediately to the left of the varnished strip, such as 7: “Right, seven!”
- “If I want to make a ten, I wonder what I would have to cover!”
- Start at 10 on the board, and count the eight boxes to the right of 10.
- “Eight.”
- Point to the bottom of the varnished strip stair, and count one strip at a time until you reach the 8th strip.
- Place the 8th strip over the red numbers aligned right.
- “See, there’s 10!”
- Put the strip back and replace it with a random strip on the board, for example: “We now have thirteen!”
- Place a blue strip adjacent to the varnished strip, in this example, the blue strip of 5.
- “Thirteen, take away five is…eight! There’s eight!”
- Repeat this process with random varnished strips and taking away a number a few more times, then ask the child to do it.

##### Presentation

- Continue from the introduction on the same day.
- “I have some paper we can use for this, let me show you!”
- Show the child the ‘subtraction table of ___” papers.
- Ask the child to bring a few papers, one underlay and one pencil on a tray.
- “We’re going to start with the table of 18. Can you write an 18 in this blank spot here?”
- “If we are doing eighteen, do we need to cover anything up with a varnished strip? No, we don’t, there’s nothing to cover!”
- Let’s do the first problem. It says eighteen minus nine; you can do that!”
- Wait for the child to place a blue strip of 9 aligned to the right on the board.
- “What’s the answer? Right, nine!”
- Wait for the child to write the answer down.
- “Eighteen minus nine equals nine!”
- “What’s next? Eighteen minus eight, go ahead!”
- Repeat this process for eighteen minus eight.
- Once the child discovers the answer, say:

“Oh, that answer is right! But you know what? We don’t need answers that are in red! So, we are all done with the table of 18!” - Repeat this process for the table of 17 and the table of 16.
- “We can use subtraction chart 1 to check the answers!”
- Once the child is done, show him how to place the varnished strips back, longest first on the wall of the box which is closest to you.

##### Ways to Take Away From a Number

- Invite the child to the lesson.
- Ask the child to set up the subtraction strip board and strips on the table.
- Place the red strips on the table in stair formation:

“We are going to use the red strips too this time!” - Choose a larger number, such as seven.
- Place a varnished strip to the right of the seven, covering the numbers 8 through 18, leaving numbers 1 through 7.
- “We’re going to make seven in different ways, then I will show you something new!”
- Place a red strip of 7 in the first row of the board, aligned to the left.
- Place a red strip of 6 in the next row.
- “What do we need to make this six into a seven? Right, a one!”
- Place a blue strip of 1 next to the red strip of 6.
- Repeat this process with a red strip of 5 and a blue strip of 2, and so on until all the combinations are made.
- Bring two papers, underlays, and pencils on a tray.
- “Let’s take away some numbers!”
- Start with the row at the bottom, which in this case is a blue strip of 7.
- “Seven take away seven” slide the blue strip of 7 down to the bottom row of the board “is what?” Right, it’s nothing!”
- Write on your paper: 7 – 7 = 0 Ask the child to write it on his paper too.
- “Seven take away six” slide the blue strip of six down to the 2nd to last row of the board “is what? Right, one!”
- Write on your paper: 7 – 6 = 1 Ask the child to write it on his paper too.
- Repeat this process for the rest of the strips, handing it off to the child for seven minus five.
- Once the child is done:

“We can check with subtraction chart 1!” - Ask the child to do another number and let him work independently.

#### Exercises

Child works independently from 18 down to 1.

#### Direct Purpose

To work though all the subtraction combinations leading to memorization. To see that addition and subtraction are the reverse processes.

#### Control of Error

Subtraction Chart 1

#### Age

5 1/2 to 6