1. Mental Subtraction In your head, compute: 65 - 28 71 - 39 80 - 46 Explain what you did so that a second grader will understand.

2. Subtraction Vocabulary: A - B = C A is the minuend B is the subtrahend C is the difference

3. Try this: Is A - B = B - A? 6 - 4 = 4 - 6? What is true of 6 - 4 and 4 - 6? One of the worst things we can say: “You can’t take a bigger number away from a smaller number.” Or “You have to put the bigger number first (in the minuend).”

4. Pictorial Models Take away Comparison

5. Number Line Model 7 - 9 = -2 Why is it important to start at 0? -2 -1 0 1 2 3 4 5 6 7 8 9 10

6. Four related facts If 9 - 4 = 5, then 9 - 5 = 4, 4 + 5 = 9, and 5 + 4 = 9 You try: If 74 - 61 = 13, then …

7. Mental Subtraction Not as obvious as mental addition 65 - 28 Break apart the second number (65 - 20) - 8 Adding up 28 + 30 = 58, 58 + 7 = 65, 30 + 7 Compensation (65 + 2) - (28 + 2), 67-30 Compatible Numbers (65 – 25, 40-3)

8. The name of the strategy is not important… You try… and be ready to explain how you did it. Are there other ways? 73 - 82 97 - 39 301 - 293

9. Regrouping Show a diagram for 302 - 84

10. Start with…

11. Now, indicate what you are subtracting 302 - 84: Let red be the part you take away

12. You try: draw pictures 58 - 37 47 - 29

13. Subtraction the way you learned it… 6 7 8 - 3 9 2 2 8 6 5 1 Why did you cross out the 6? Why did you put a little “1” next to the 7? Can you show this with pictures?

14. Here are three other ways to think about subtraction Explain why this works--use pictures or manipulatives 9 8 4 - 3 6 8 6 1 6 7 1

15. Why does this work? This way worked because 984 - 368 is the same as adding 10 to both numbers: 984 + 10 = 980 + 4 + 10 = 980 + 14 368 + 10 = 360 + 8 + 10 = 370 + 8

16. Use drawings or manipulatives to explain… Here is yet another one. 3 2 6 - 2 9 4 2 -7 0 1 0 0 100 - 70 + 2 = 132

17. A real problem Place the digits 1, 2, 3, 6, 7, 8 in the boxes to obtain: Greatest sum Least sum Greatest difference Least difference