1. I can multiply decimals
Multiplying Decimals
I can multiply decimals

2. To Multiply: You do not line up the factors by the decimal.
Instead, place the number with more digits on top.
Line up the other number underneath, at the right.
Multiply
Count the number of decimal places (from the right) in each factor.
Use the total number of decimal places in your two factors to place the decimal in your product.

3. Multiply Decimals Step 1 --- Ignore the decimal
Step Rewrite the problem
Step Multiply the problem
Step Underline all numbers to the right of decimal(s) in original problem
Step Move decimal that many places left in your answer

4. Steps 1 & 2 Ignore decimals and rewrite the problem 4 . 5 6 X 0 . 3 9
4 5 6
x 3 9

5. Step 3 Multiply problem without decimals 4 5 6 x 3 9 4 1 0 4

6. x 0. 3 9 --------------- Step 4
Underline numbers to the right of decimals in original problem
4. 5 6 x
( 4 places)

7. Step 5
Move decimal that many places left in answer
4 5 6 x
( 4 places)

8. I can multiply decimals
Multiplying Decimals
I can multiply decimals

9. Multiplying Decimals
You can use graph paper to model decimal multiplication. Here is a model of the product of 0.7 and 0.3.
0.7
0.3 1
Because 21 of the 100 squares are shaded, 0.7  0.3
= 0.21. 1
NOTE
BOOK
Multiplying Decimals
Multiply decimals as you would whole numbers. Then place the decimal point in the product. The number of decimal places in the product is equal to the sum of the number of decimal places in the factors.
Words
Numbers
0.7  0.3 = 0.21

10. 1 Multiplying Decimals  Multiplying Decimals 5.82 –––––– 0.41 –––
EXAMPLE 1
Multiplying Decimals
5.82
2 decimal places
–––––– 
0.41
––– +
2 decimal places
5 8 2
–––––––
4 decimal places 4 3 1 2

11. Example: 5.63 x 3.7 1 4 2
5.63
two x
3.7
one 1 1 39 4 1 1 + 16 8 9 2 . 8 3 1
three

12. 2 Multiplying Decimals  Multiplying Decimals 6.45 –––––– 18 ––– 5160
EXAMPLE 2
Multiplying Decimals
6.45
2 decimal places
––––––  18
0 decimal places
––– +
5160
6450
––––––
116.10
2 decimal places
After you place the decimal point, you can drop any zeros at the end of an answer.
ANSWER
6.45  18 = 116.1
CHECK
Because 6  20 = 120, the product is reasonable.

13. 2 Multiplying Decimals   Multiplying Decimals 6.45 1.273 –––––– 18
EXAMPLE 2
Multiplying Decimals
6.45
2 decimal places
1.273
3 decimal places
––––––  18
0 decimal places
––– +
––––––– 
0.06
2 decimal places
––– +
5160
0.0
7638
5 decimal places
645
––––––
116.10
2 decimal places
Write a zero before the 7 as a placeholder so that the number has 5 decimal places.
After you place the decimal point, you can drop any zeros at the end of an answer.
ANSWER
6.45  18 = 116.1
ANSWER
1.273  0.06 =
CHECK
CHECK
Because 6  20 = 120, the product is reasonable.
Because 1  = 0.06, the product is reasonable.

14. 3 Multiplying Decimals Multiplying Decimals to Find Area
EXAMPLE 3
Multiplying Decimals to Find Area
Central Park Central Park is a rectangular park about 2.5 miles long and about 0.5 mile wide. What is the area of Central Park?
SOLUTION
A = l w
Write formula for area of a rectangle.
= l w
2.5
(0.5)
Substitute 2.5 for l and 0.5 for w.
= 1.25
Multiply.
ANSWER
The area of Central Park is about 1.25 square miles.

15. Showdown
.31 x .7 =
.217
.9 x .03 =
.027
2 x .006 =
.012
.66 x 2.52 =
1.6632