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Mental Math for Addition and Subtraction

Mental Math for Addition and Subtraction. Lesson 2.1. Commutative Property. Associative Property. Addition Properties. Numbers can be added in any order. Addends can be grouped in different ways, resulting in the same sum. 64 + 78 = 78 + 64. (14 + 54) + 6 = 14 + (54 + 6).

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Mental Math for Addition and Subtraction

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  1. Mental Math for Addition and Subtraction Lesson 2.1

  2. Commutative Property Associative Property Addition Properties • Numbers can be added in any order. • Addends can be grouped in different ways, resulting in the same sum. 64 + 78 = 78 + 64 (14 + 54) + 6 = 14 + (54 + 6)

  3. Example - Find the Sum First, let’s rearrange the addends. 5 + 9 + 21 + 25 5 + 25 + 9 + 21 Commutative Property (5 + 25) + (9 + 21) Associative Property 30 + 30 Use mental math 60 So,the sum is 60

  4. Compensation • You can change one number to a multiple of 10 (make it end in 0) and then adjust the other addend to keep the balance. 18 + 24 = (18 + 2) + (24 - 2) Add 2 to make 20 Subtract 2 to balance = 20 + 22 = 42

  5. Using Compensation to Subtract • Make the second number a multiple of 10 (multiples of 10 are numbers that end in 0). • Subtract the same amount from both numbers. 57 - 33 = (57 - 3) - (33 - 3) Make 33 a multiple of 10 by subtracting 3. Take away the same amount from the first number. = 54 - 30 = 24

  6. Homework Time!

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