Unit 3 Day 3: Adding & Subtracting Decimals

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Unit 3: The World of Decimals

Day 3: Adding & Subtracting

Quick Shopping Trip!

Snack

$1.25

+

Drink

$2.50

How much will you spend in total?

Today's Objectives

The Golden Rule of Decimals

When you add or subtract decimals, you MUST line up the decimal points!

Why Does It Matter?

Lining up the decimals ensures you are adding and subtracting digits with the same place value. (ones with ones, tenths with tenths, etc.)

WRONG \(\times\)

  1.2
+ 0.35
------

RIGHT \(\checkmark\)

  1.20
+ 0.35
------

I Do: Adding Decimals

Let's solve 3.75 + 1.8.

3.75

+ 1.80 \(\leftarrow\) Annex a zero to line everything up!

5.55

Just bring the decimal point straight down in your answer!

I Do: Subtracting Decimals

Let's solve 8.5 - 2.35.

8.50 \(\leftarrow\) You MUST annex a zero here!

- 2.35

6.15

You may need to borrow, just like with whole numbers.

We Do: Let's Try Together

Solve: 12.4 + 3.85

  12.40
+  3.85
-------

Solve: 9.2 - 1.7

  9.2
- 1.7
-----

You Do: Practice with a Partner

Try these two problems on your whiteboards.

Problem 1:

Solve:

25.5 + 8.92

Problem 2:

Solve:

14 - 3.5

Independent Practice

Try these problems on your own.

Green Level

Solve: 0.6 + 0.3

Yellow Level

Solve: 12.5 - 4.25

Red Level

You have $20.00. You buy a sandwich for $8.50 and a drink for $2.75. How much change do you get back?

Exit Ticket

On a piece of paper, solve these two problems.

1. Solve: 5.8 + 2.45

2. Solve: 18 - 6.75

Unit 3: The World of Decimals

Day 3: Adding & Subtracting Decimals - Notes

Today's Objectives

The Golden Rule

When you add or subtract decimals, you MUST !

This makes sure you are adding and subtracting digits with the same place value.

WRONG \(\times\)

  1.2
+ 0.35
------

RIGHT \(\checkmark\)

  1.20
+ 0.35
------

I Do: Teacher Examples

Addition: 3.75 + 1.8

  3.75
+ 1.80
------

Subtraction: 8.5 - 2.35

  8.50
- 2.35
------

We Do: Guided Practice

Addition: 12.4 + 3.85 =

Subtraction: 9.2 - 1.7 =

You Do: Partner Practice

Addition: 25.5 + 8.92 =

Subtraction: 14 - 3.5 =

Independent Practice

Green: Solve: 0.6 + 0.3 =

Yellow: Solve: 12.5 - 4.25 =

Red: You have $20.00. You buy a sandwich for $8.50 and a drink for $2.75. How much change do you get back?

Exit Ticket

1. Solve: 5.8 + 2.45

2. Solve: 18 - 6.75