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Decimals and Percents
U.S. Traditional Multiplication: Decimals
You can also use U.S. traditional multiplication with decimals.
Here are two ways to use this method for multiplying decimals.
Method 1
One way to use U.S. traditional multiplication to multiply decimals
is to multiply them as though they were whole numbers and then use
estimation to place the decimal point.
Step 1: Multiply as though both factors were whole numbers.
Step 2: Estimate the product.
Step 3: Use your estimate to place the decimal point in the answer.
6 4.79 ?
Step 1: Multiply as though the factors were whole numbers.
Step 2: Estimate the product.
4
5
4 7 9
6
2 8 7 4
6 4.79 6 5 30
Step 3: Use the estimate to place the decimal point in the answer.
The estimate is 30, so place the decimal point to make the
answer close to 30: 28.74 is close to 30.
6 4.79 28.74
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Decimals and Percents
Method 2
The second method for multiplying decimals is useful when
there are many decimal places in the factors and it becomes
harder to estimate the answer.
Step 1: Multiply as though both factors were whole numbers.
Step 2: Count the total number of places to the RIGHT of the
decimal point for both factors.
Step 3: Place the decimal point so that you have the same
number of decimal places as the total in Step 2.
0.065 0.032
When people talk about the
number of “decimal places”
in a number, they usually
mean the number of places
to the right of the decimal
point. So, for example, we
say the number 11.732
has three decimal places.
The number 45.06 has two
decimal places. The
number 0.0078 has four
decimal places.
1
1
Step 1: Multiply as though the factors were whole numbers.
Step 2: Count the total number of decimal places in both factors.
0.065 has 3 decimal places; 0.032 has 3 decimal places.
There are 6 decimal places in all.
3
6
1 6
1 9 2
2 0 8
2
5
0
0
0
Step 3: Place the decimal point 6 places from the right.
Insert 0s as needed.
0.065 0.032 0.002080
Multiply.
1. 74.5 * 3 ?
2. 4.86 * 0.36 ?
3. 0.073 * 2.07 ?
Check your answers on page 424A.
60D
4. 0.05 * 6.3 ?