Topic 2, Lesson 1 Mental Math to Add

Topic 2, Lesson 1 Mental Math to Add

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Let's do a little

Review of What We Know first!

Poll

A process that determines which multiple of 10, 100, 1,000, and so on a number is closest to is called what?

• Estimating

• Rounding

• Adding

• Place Value

Multiple Choice

A number sentence that uses the equal sign (=) to show two expressions have the same value is a _________?

• equation

• estimate

• period

• rounding

Topic 2-1: Mental Math: Find Sums and Differences

There are a number of strategies we can use to help us add & subtract easier. This will lead to adding in our heads, or mental math.

We can use:

• Compensation

• Commutative Property of Addition

• Associative Property of Addition

• Break Apart

Compensation

You can use compensation, a strategy that involves choosing numbers close to the actual numbers in a problem to make the computation easier, and then adjusting the answer for the numbers chosen, to find sums and differences.

Example of Compensation

• 157 + 43 + 142

• Change 157 to 160 (by taking 3 ones from 43)

• Now you have 160 + 40 + 142

• 160 + 40 = 200

• 200 + 142 is much easier! 342

Commutative Property of Addition

You can use the Commutative Property of Addition, a strategy that states that numbers can be added in any order and the sum remains the same. For example: 15 + 35 = 35 + 15

Fun Facts

• Commutative is like the word Community, where order should not matter. We are all friends in a community!

• Wearing shoes, gloves, or putting on socks are examples of Commutative Property, because the order in which you wear them is not important!

Associative Property of Addition

You can use the Associative Property of Addition, a strategy that states addends can be regrouped and the sum remains the same. We typically use this when addends are in parentheses.

Example of Associative Property

• 157 + (43 + 142)

• Change the parentheses to go around 157 and 43

• Now you have (157 + 43) + 142

• 157 + 43 = 200

• 200 + 142 is much easier! 342

Break Apart

You can use the break apart to break up one addend in an equation into more manageable parts. For example: 157 + 43 can be broken apart in ones, tens, and hundreds.

Example of Break Apart

• 157 + 43

• Break into ones, tens, and hundreds

• Now you have (100) + (50 + 40) + (7 + 3)

• 100 + 90 + 10 = 200

• Much easier!

Let's try some.

Get a piece of paper to help you

in case you cannot solve these in your head.

Multiple Choice

Sheri adds 135 + 1,048 using mental math. What properties and strategies can Sheri use to find the sum?

• Shari can use break apart to make 1,048 (1,043 + 5) so she can easily add 135 + 5 first.

• Shari can use compensation and make 1,048 into 1,050 (taking 2 from 135) so she can add 1,050 + 133.

Multiple Choice

Use a strategy of your choice to solve:

2,314 + 5,525 =

• 7,939

• 7,840

• 7,839

Multiple Choice

What strategy might make sense to solve 6,794 - 1,058 ?

• Use 6,794 pennies. Remove 1,058 pennies

  and count how many remain.

• Use break apart to subtract

  6,700 - 1,000 and 94 - 58

Multiple Choice

Use any strategy to solve

6,794 - 1,058 =

• 5,736

• 5,746

• 5,756

Next Steps

Go to Schoolwork to Open EdPuzzle.

Don't forget to Mark Done when you finish EdPuzzle.

Then Open Keynote Practice Buddy.

Don't forget to Submit when you finish.