Lesson 2-1 Mental Math

1) Solve & Share ​

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​2) Key Vocabulary (commutative property, associative property, compatible numbers, and compensation)

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​3) Convince Me ! for checking on your understanding

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​4) Guided Practice working in groups

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​5) Independent Practice working in groups

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​6) Practice Buddy working independently

• ​Three pieces of software cost $20.75, $10.59, and $18.25. What is the total cost of the software? Use mental math to solve.

• ​Three pieces of software cost $20.75, $10.59, and $18.25. What is the total cost of the software? Use mental math to solve.

Objective ​

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I can use mental math to solve addition and subtraction problems. 

​Essential Question

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How can we manipulate numbers to help us solve problems using mental math?

​You can use the commutative property to rearrange the addends, so $4.75 + $1.25 + $1.50 will be the new order of the numbers. I can add $0.75 + $0.25 which is $1. Now, $4 + $1 + $1 = $6. Finally, $6 + $1.50 = $7.50.

Commutative property - focus on the order of numbers does not matter

​$11.45 + $ 3. 39 = $3.39 + $11.45 Think : movement or commuting

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​Associative property - focus on the groupings of numbers does not matter

​($11.45 + $3.39) + $9.55 = $11.45 + ($3.39 + $9.55) Think: associating yourself with a group

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​Compatible number - numbers that are easy to calculate mentally

​$11.45 + $9.55 = $21 and $21 + $3.39 = $24.39

​Complete the 2-1 Practice Buddy. You will work for the remaining time in class. When done, work on Prodigy.