Download Multiplying Decimals

3-4 Multiplying Decimals
Warm Up
Problem of the Day
Lesson Presentation
Lesson Quizzes
3-4 Multiplying Decimals
Warm Up
Multiply.
1. 87  320
27,840
2. 943  800
754,400
3. 3,806  10
4. 1,207  100
38,060
120,700
5. 72  196
14,112
6. 120  523
62,760
3-4 Multiplying Decimals
Problem of the Day
Carmen and Rita sold homemade
oatmeal cookies. After 8 days, they had
sales totaling $70. Each day, their sales
were $0.50 higher than the previous
day. What were their sales on the first
day?
$7.00
3-4 Multiplying Decimals
Sunshine State Standards
MA.6.A.1.2 Multiply…decimals efficiently.
MA.6.A.1.3 Solve real-world problems
involving multiplication…of…decimals.
Also MA.6.A.1.1
3-4 Multiplying Decimals
Additional Example 1: Science Application
Something that weighs 1 lb on Earth weighs
0.17 lb on the Moon. How much would a 4 lb
dumbbell weigh on the Moon?
4  0.17
17
 4
68
Multiply 4 by 0.17, since 1 pound on
Earth is 0.17 pound on the Moon.
Multiply as you would with whole
numbers.
3-4 Multiplying Decimals
Additional Example 1 Continued
Something that weighs 1 lb on Earth weighs
0.17 lb on the Moon. How much would a 4 lb
dumbbell weigh on the Moon?
Place the decimal point by adding the number
of decimal places in the numbers multiplied.
0.17
 4
0.68
2 decimal places
+ 0 decimal places
2 decimal places
A 4 lb dumbbell on Earth weighs 0.68 lb on the Moon.
3-4 Multiplying Decimals
Check It Out: Example 1
Something that weighs 1 lb on Earth weighs
0.17 lb on the Moon. How much would a 7 lb
dumbbell weigh on the Moon?
7 × 0.17 = 1.19 lb
3-4 Multiplying Decimals
Helpful Hint
You can use a decimal grid to model multiplication
of decimals.
3-4 Multiplying Decimals
Additional Example 2A: Multiplying a Decimal by
a Decimal
Find the product.
0.3  0.4
Multiply. Then place the decimal point.
0.3
1 decimal place
 0.4
+ 1 decimal place
0.12
2 decimal places
3-4 Multiplying Decimals
Additional Example 2B: Multiplying a Decimal by
a Decimal
0.07  0.8
0.07  1 = 0.07 Estimate the product. 0.8 is
close to 1.
0.07
 0.8
0.056
Multiply. Then place the
decimal point.
2 decimal places
+ 1 decimal place
3 decimal places; use a
placeholder zero
0.056 is close to the estimate of 0.07. The
answer is reasonable.
3-4 Multiplying Decimals
Additional Example 2C: Multiplying a Decimal by
a Decimal
1.34  2.5
13=3
1.34
 2.5
670
2680
3.350
Estimate the product. Round
each factor to the nearest
whole number.
Multiply. Then place the
decimal point.
2 decimal places
+ 1 decimal place
3 decimal places
3.350 is close to the estimate of 3. The answer
is reasonable.
3-4 Multiplying Decimals
Check It Out: Example 2
Find the product.
3.80 × 3.3
3.80
 3.3
1140
11400
12.540 = 12.54
3-4 Multiplying Decimals
Additional Example 3A: Evaluating Decimal
Expressions
Evaluate 5x for the value of x.
x = 3.062
5x = 5(3.062)
Substitute 3.062 for x.
3.062
3 decimal places
5
+ 0 decimal places
15.310
3 decimal places

Remember!
These notations all mean multiply 3 times x.
3•x
3x
3(x)
3-4 Multiplying Decimals
Additional Example 3B: Evaluating Decimal
Expressions
Evaluate 5x for the value of x.
x = 4.79
5x = 5(4.79)

Substitute 4.79 for x.
4.79
2 decimal places
5
+ 0 decimal places
23.95
2 decimal places
3-4 Multiplying Decimals
Check It Out: Example 3
Evaluate 5x for the value of x.
x = 6.22
5(6.22) = 31.10 = 31.1