# Week 4 In your head, compute: 65 - 28 71 - 39 80 - 36 Explain what you did so that a second grader will understand.

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Week 4 In your head, compute: 65 - 28 71 - 39 80 - 36 Explain what you did so that a second grader will understand.

Announda Don’t forget to initial attendance sheet! Exam 1 is one week from today! Some practice questions will be posted online. Monday Office Hour: Rescheduled to 12:00- 1:00pm in MATHE LOBBY Subtraction Continued

Pictorial Models Take away Judy has 7 crosses and gives 2 to Jake. How many does Judy have left?

Pictorial Models Comparison Amy has 7 crosses and Andy has 2. How many more crosses than Andy does Amy have?

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

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 …

Subtraction & Addition do we NEED both?

Mental Subtraction 65 – 28

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) Compatible Numbers (65 - 25, add back 3)

Exploration 3.1 (p. 50) 3abck 4ac

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