1-2 Adding and Subtracting DecimalsDo Now: 1.Vocabulary Review: What is place value?Write the value of the digit 8 in each number.2. 0.3853. 1.8794. 812.45. 0.23

Homework Answers 1-1

Guided Problem Solving 1-1

• 1. eight postcards; $.59 each; $2 total• 2. Estimate the total cost of eight postcards and

stamps.• 3. The product of 8 and $.59• 4. $.60• 5. $4.80• 6. $2• 7. $4.80 1 $2 5 $6.80• 8. yes, 8 3 0.59 1 2 5 6.72, which is close to

6.80• 9. about $4.80

Addition PropertiesCommutative Property of Addition: Numbers can be added in any order.

Example: 5+3=3+5

Associative Property of Addition: Numbers can be grouped in any way.

Example: (7+3) + 5 = 7 + (3+5)

Identity Property of Addition: The sum of any number and zero is that number.

Example: 18 + 0 = 18

The Basic Steps:

• Line up the numbers by the decimal point.

• Fill in missing places with zeroes.

• Add or subtract.

Mental Math• Addition properties can be used to quickly

find sums.• Example: What is 52 + 12.4 + 48?• We can use an addition property to

quickly add this. • First, change the order: 52 + 48 + 12.4

________________ property.• Then, group the first two addends

together: (52 + 48) + 12.4________________ property

• 100 + 12.4 = 112.4

commutative

associative

Why Estimate?

• Before adding your numbers, use one of the estimating strategies you learned to see what the answer should be close to.

• When you add, you can see if your answer is reasonable.

Example: 28.9 + 13.31

This can estimate to 29 + 13, which is 42. (Rounding)

28.913.31+

0

42.21The answer is close to the estimate.

Subtracting

• The addition properties do not work for subtraction.

• Estimate and subtract to find what the answer is close to.

• Subtract to find the real answer.

Subtracting Across Zeroes• If you have several zeroes

in a row, and you need to borrow, go to the first digit that is not zero, and borrow.

• All middle zeroes become 9’s.

• The final zero becomes 10.

Example: 15 – 9.372

159.372-

.0004 9 9 10

826.

0 1

5

Title of Band Selections

Min

“76 Trombones”

3.12

“Stars and Stripes Forever”

3.52

“Born in the U.S.A.”

4.65

“America the Beautiful”

3.63

“Yankee Doodle Dandy”

2.5

“The Star-Spangled Banner”

4.5

1. A marching band has 6.5 minutes to perform at a football game. Does the band have enough time to play “America the Beautiful” and Yankee Doodle Dandy”?

1. How long will the band take to play “Stars and Stripes Forever” and “The Star Spangled Banner”?

Title of Band Selections

Min

“76 Trombones”

3.12

“Stars and Stripes Forever”

3.52

“Born in the U.S.A.”

4.65

“America the Beautiful”

3.63

“Yankee Doodle Dandy”

2.5

“The Star-Spangled Banner”

4.5

1. A marching band has 6.5 minutes to perform at a football game. Does the band have enough time to play “America the Beautiful” and Yankee Doodle Dandy”?

The band will play for 6.13 minutes. The band will have enough time.

2. How long will the band take to play “Stars and Stripes Forever” and “The Star Spangled Banner”?

8.02 min