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# Subtraction Strip Board

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4. Memorization
5. Subtraction Strip Board

#### 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
1. Invite the child to the lesson.
2. Show him the subtraction strip board.
3. Point out the box of varnished strips
4. Ask him to bring the board to the table.
5. Bring the red and blue strips, underlay, and box of varnished strips.
6. Ask the child to place the blue strips and how there are no numbers on them.
7. Briefly talk about the varnished strips and how there are no numbers on them.
8. Show the child where to place the varnished strips, longest first, at the top right of the table.
9. Place the second longest strip underneath the previous strip; strips aligned to the right.
10. Ask the child to place the rest of the varnishes strips.
11. “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.”
12. “We can use these strips to cover up the numbers.”
13. Place a varnished strip on the red numbers, aligned right, emphasize the right alignment.
14. “What number is left?”
15. Point to the number that’s immediately to the left of the varnished strip, such as 7: “Right, seven!”
16. “If I want to make a ten, I wonder what I would have to cover!”
17. Start at 10 on the board, and count the eight boxes to the right of 10.
18. “Eight.”
19. Point to the bottom of the varnished strip stair, and count one strip at a time until you reach the 8th strip.
20. Place the 8th strip over the red numbers aligned right.
21. “See, there’s 10!”
22. Put the strip back and replace it with a random strip on the board, for example: “We now have thirteen!”
23. Place a blue strip adjacent to the varnished strip, in this example, the blue strip of 5.
24. “Thirteen, take away five is…eight! There’s eight!”
25. Repeat this process with random varnished strips and taking away a number a few more times, then ask the child to do it.
##### Presentation
1. Continue from the introduction on the same day.
2. “I have some paper we can use for this, let me show you!”
3. Show the child the ‘subtraction table of ___” papers.
4. Ask the child to bring a few papers, one underlay and one pencil on a tray.
5. “We’re going to start with the table of 18. Can you write an 18 in this blank spot here?”
6. “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!”
7. Let’s do the first problem. It says eighteen minus nine; you can do that!”
8. Wait for the child to place a blue strip of 9 aligned to the right on the board.
9. “What’s the answer? Right, nine!”
10. Wait for the child to write the answer down.
11. “Eighteen minus nine equals nine!”
12. “What’s next? Eighteen minus eight, go ahead!”
13. Repeat this process for eighteen minus eight.
14. 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!”
15. Repeat this process for the table of 17 and the table of 16.
16. “We can use subtraction chart 1 to check the answers!”
17. 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
1. Invite the child to the lesson.
2. Ask the child to set up the subtraction strip board and strips on the table.
3. Place the red strips on the table in stair formation:
“We are going to use the red strips too this time!”
4. Choose a larger number, such as seven.
5. Place a varnished strip to the right of the seven, covering the numbers 8 through 18, leaving numbers 1 through 7.
6. “We’re going to make seven in different ways, then I will show you something new!”
7. Place a red strip of 7 in the first row of the board, aligned to the left.
8. Place a red strip of 6 in the next row.
9. “What do we need to make this six into a seven? Right, a one!”
10. Place a blue strip of 1 next to the red strip of 6.
11. Repeat this process with a red strip of 5 and a blue strip of 2, and so on until all the combinations are made.
12. Bring two papers, underlays, and pencils on a tray.
13. “Let’s take away some numbers!”
14. Start with the row at the bottom, which in this case is a blue strip of 7.
15. “Seven take away seven” slide the blue strip of 7 down to the bottom row of the board “is what?” Right, it’s nothing!”
16. Write on your paper: 7 – 7 = 0 Ask the child to write it on his paper too.
17. “Seven take away six” slide the blue strip of six down to the 2nd to last row of the board “is what? Right, one!”
18. Write on your paper: 7 – 6 = 1 Ask the child to write it on his paper too.
19. Repeat this process for the rest of the strips, handing it off to the child for seven minus five.
20. Once the child is done:
“We can check with subtraction chart 1!”
21. 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

5 1/2 to 6