1. Comparative Relational Thinking
Subtraction

2. How do you know the equation is true?
Let’s Review
Are these equations true or false?
6 - 4 = 7 - 5
7 - 1 = 8
How do you know the equation is true?
Talk me through your thinking…
FALSE
TRUE

3. How did you solve the equation?
Let’s Review
Fill in the blank
12 - 3= 15 - __
17 - __=
= n - 5
How did you solve the equation?
How did you solve? 6
n = 9 2

4. What strategies do you use to subtract?
Let’s Review
What strategies do you use to subtract?
Count Back
Break Apart
Friendly Numbers
Add up
Think addition

5. Why do Friendly Numbers work?
17 - 8
Let’s think about an easy problem. 1 2 3 4 5 6 7 8 9
I will use the think addition strategy first!
17 = 8 + ___
= 9

6. Why do Friendly Numbers work?
+3 = 11
+3 = 20
17 - 8
Let’s think about adding the same value. 1 2 3 4 5 6 7 8 9
If I move 17 to 20… to keep a difference of 9… 8 moves to 11
I can keep a constant difference.
= 9
= 9

7. Why do Friendly Numbers work?
-7 = 1
-7 = 10
17 - 8
Let’s try subtracting the same value. 1 2 3 4 5 6 7 8 9
If I move 17 to 10… to keep a difference of 9… 8 moves to 1
I can keep a constant difference.
= 9
= 9

8. Think about using Friendly Numbers and a Constant Difference to subtract…
+3 = 49
+3 = 30
Then I subtract 49 and 30 to find the difference.
= 19
If I add 3 to 27, then I need to add 3 to 46… = 49
I like to subtract with multiples of 10, so…
I need to add 3 to 27 to make 30.
I made a ten to count up to solve the subtraction.
The answer is 19!

9. If you add 3 to the 18, you need to add 3 to the 67.
Using Friendly Numbers and a Constant Difference in Comparative Relational Thinking
+3 = 70
+3 = 21
Let’s determine if this equation is TRUE or FALSE. = = =
If you add 3 to the 18, you need to add 3 to the 67.
Using friendly numbers, but instead of making a 10, I would change one number on one side to match the other side.
It does not matter which number is matched…
BOTH numbers do NOT match, so it is FALSE!!!
is 70.
So, I can add 3 to 18 to get 21?
I don’t want to solve… I just want to COMPARE

10. Using Friendly Numbers and a Constant Difference in Comparative Relational Thinking to find an Unknown
Find the unknown.
=  - 480
+13
+ 13
756
The unknown is equal to 756!!
480
First, I change the value of one number to be the same as the number on the other side of the equal sign.
Then I would add or subtract the same value to the other number to keep a constant difference.

11. True or False? =
true

12. True or False?
5, ,782 = 5, ,795
false

13. Find the missing number.
= 250
346 – 37 = 309
Answer 309

14. Find the missing number.
9, ,342 = ___ - 8,348
9,573