Mental Subtraction In your head, compute: Explain what you did so that a second grader will understand.

Subtraction in Alphabitia CBD − ADC

Let’s subtract CBD - ADC= CBD is CBD - ADC is

You Try… D00 -B0B B0A - BD

You try: Use manipulatives or drawings to explain BBC - A0C A0A0D - BCA

Exploration 3.1 Part 2 #1 Write two subtraction story problems. One must require regrouping. Share these in your group. #2 What interpretations of subtraction can you describe?

Estimation Actual answer: 235 Ways to estimate: = = = = = 250

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

Try this: Is A - B = B - A? = 4 - 6? What is true of 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).”

Pictorial Models Take away Comparison

Number Line Model = -2 Why is it important to start at 0?

Four related facts If = 5, then = 4, = 9, and = 9 You try: If = 13, then …

Mental Subtraction Subtract mentally: 65 − 28

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

You try… and be ready to explain how you did it. Are there other ways?

Regrouping Show a diagram for

Start with…

Now, indicate what you are subtracting : Let red be the part you take away

You try: draw pictures

Subtraction the way you learned it… Why did you cross out the 6? Why did you put a little “1” next to the 7? Can you show this with pictures?

Here are three other ways to think about subtraction Explain why this works--use pictures or manipulatives

Why does this work? This way worked because is the same as adding 10 to both numbers: = = = =

Use drawings or manipulatives to explain… Here is yet another one = 132

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