1.4: Multiplying and Dividing Decimals

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Learning Objectives

Use multiplication and division when evaluating expressions with decimals

Multiplying and dividing decimals can become complex, but we’re going to start with some basic multiplication by ten to warm-up.

Multiply by Powers of $10$

In many fields, especially in the sciences, it is common to multiply decimals by powers of $10$ . Let’s see what happens when we multiply $1.9436$ by some powers of $10$ .

The top row says 1.9436 times 10, then 1.9436 times 100, then 1.9436 times 1000. Below each is a vertical multiplication problem. These show that 1.9436 times 10 is 19.4360, 1.9436 times 100 is 194.3600, and 1.9436 times 1000 is 1943.6000.
Look at the results without the final zeros. Do you notice a pattern?

$\begin{array}{ccc}1.9436\left(10\right)\hfill & =& 19.436\hfill \\ 1.9436\left(100\right)\hfill & =& 194.36\hfill \\ 1.9436\left(1000\right)\hfill & =& 1943.6\hfill \end{array}$

The number of places that the decimal point moved is the same as the number of zeros in the power of ten. The table below summarizes the results.

Multiply by	Number of zeros	Number of places decimal point moves
$10$	$1$	$1$ place to the right
$100$	$2$	$2$ places to the right
$1,000$	$3$	$3$ places to the right
$10,000$	$4$	$4$ places to the right

We can use this pattern as a shortcut to multiply by powers of ten instead of multiplying using the vertical format. We can count the zeros in the power of $10$ and then move the decimal point that same of places to the right.

So, for example, to multiply $45.86$ by $100$ , move the decimal point $2$ places to the right.

45.86 times 100 is shown to equal 4586. There is an arrow from the decimal going over 2 places from after the 5 to after the 6.
Sometimes when we need to move the decimal point, there are not enough decimal places. In that case, we use zeros as placeholders. For example, let’s multiply $2.4$ by $100$ . We need to move the decimal point $2$ places to the right. Since there is only one digit to the right of the decimal point, we must write a $0$ in the hundredths place.

2.4 times 100 is shown to equal 240. There is an arrow from the decimal going over 2 places from after the 2 to after the 0.

Multiply a decimal by a power of $10$

Move the decimal point to the right the same number of places as the number of zeros in the power of $10$ .
Write zeros at the end of the number as placeholders if needed.

In the following video we show more examples of how to multiply a decimal by 10, 100, and 1000.

Thumbnail for the embedded element "Multiply Decimals by 10, 100, and 1000"

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example

Multiply $5.63$ by factors of

1. $10$

2. $100$

3. $1000$
[reveal-answer q=”296737″]Show Answer[/reveal-answer]
[hidden-answer a=”296737″]

Solution
By looking at the number of zeros in the multiple of ten, we see the number of places we need to move the decimal to the right.

1.
$56.3\left(10\right)$
There is $1$ zero in $10$ , so move the decimal point $1$ place to the right.
$56.3$

2.
$5.63\left(100\right)$
There are $2$ zeros in $100$ , so move the decimal point $2$ places to the right.
$563$

3.
$5.63\left(1000\right)$
There are $3$ zeros in $1000$ , so move the decimal point $3$ places to the right.
A zero must be added at the end.	$5,630$

[/hidden-answer]

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[ohm_question]146599[/ohm_question]

Multiplying Decimals

Multiplying decimals is very much like multiplying whole numbers—we just have to determine where to place the decimal point. The procedure for multiplying decimals will make sense if we first review multiplying fractions.

Do you remember how to multiply fractions? To multiply fractions, you multiply the numerators and then multiply the denominators.

So let’s see what we would get as the product of decimals by converting them to fractions first. We will do two examples side-by-side below. Look for a pattern.

	A	B
	$\left(0.3\right)\left(0.7\right)$	$\left(0.2\right)\left(0.46\right)$
Convert to fractions.	$\left({\Large\frac{3}{10}}\right)\left({\Large\frac{7}{10}}\right)$	$\left({\Large\frac{2}{10}}\right)\left({\Large\frac{46}{100}}\right)$
Multiply.	${\Large\frac{21}{100}}$	${\Large\frac{92}{1000}}$
Convert back to decimals.	$0.21$	$0.092$

There is a pattern that we can use. In A, we multiplied two numbers that each had one decimal place, and the product had two decimal places. In B, we multiplied a number with one decimal place by a number with two decimal places, and the product had three decimal places.

How many decimal places would you expect for the product of $\left(0.01\right)\left(0.004\right)?$ If you said “five”, you recognized the pattern. When we multiply two numbers with decimals, we count all the decimal places in the factors—in this case two plus three—to get the number of decimal places in the product—in this case five.

The top line says 0.01 times 0.004 equals 0.00004. Below the 0.01, it says 2 places. Below the 0.004, it says 3 places. Below the 0.00004, it says 5 places. The bottom line says 1 over 100 times 4 over 1000 equals 4 over 100,000.
Once we know how to determine the number of digits after the decimal point, we can multiply decimal numbers without converting them to fractions first. The number of decimal places in the product is the sum of the number of decimal places in the factors. The rules for multiplying positive and negative numbers apply to decimals too.

Multiply decimal numbers

Determine the sign of the product.
Write the numbers in vertical format, lining up the numbers on the right.
Multiply the numbers as if they were whole numbers, temporarily ignoring the decimal points.
Place the decimal point. The number of decimal places in the product is the sum of the number of decimal places in the factors. If needed, use zeros as placeholders.
Write the product with the appropriate sign.

example

Multiply: $\left(3.9\right)\left(4.075\right)$

Solution

$\left(3.9\right)\left(4.075\right)$
Determine the sign of the product. The signs are the same.	The product will be positive.
Write the numbers in vertical format, lining up the numbers on the right.
Multiply the numbers as if they were whole numbers, temporarily ignoring the decimal points.
Place the decimal point. Add the number of decimal places in the factors $\left(1+3\right)$ . Place the decimal point 4 places from the right.
The product is positive.	$\left(3.9\right)\left(4.075\right)=15.8925$

try it

[ohm_question]146596[/ohm_question]

In the following video we show another example of how to multiply two decimals.

Thumbnail for the embedded element "Examples 2: Multiplication of Decimals"

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Dividing Decimals

Just as with multiplication, division of decimals is very much like dividing whole numbers — we have to figure out where the decimal point must be placed.

To understand decimal division, let’s consider the multiplication problem

$\left(0.2\right)\left(4\right)=0.8$

Remember, a multiplication problem can be rephrased as a division problem. So we can write

$0.8\div 4=0.2$

We can think of this as “If we divide 8 tenths into four groups, how many are in each group?” The number line below shows that there are four groups of two-tenths in eight-tenths. So $0.8\div 4=0.2$ .

A number line is shown with 0, 0.2, 0.4, 0.6, 0.8, and 1. There are braces showing a distance of 0.2 between each adjacent set of 2 numbers.
Using long division notation, we would write

A division problem is shown. 0.8 is on the inside of the division sign, 4 is on the outside. Above the division sign is 0.2.
Notice that the decimal point in the quotient is directly above the decimal point in the dividend.

To divide a decimal by a whole number, we place the decimal point in the quotient above the decimal point in the dividend and then divide as usual. Sometimes we need to use extra zeros at the end of the dividend to keep dividing until there is no remainder.

Divide a decimal by a whole number

Write as long division, placing the decimal point in the quotient above the decimal point in the dividend.
Divide as usual.

example

Divide: $0.12\div 3$

Solution

	$0.12\div 3$
Write as long division, placing the decimal point in the quotient above the decimal point in the dividend.
Divide as usual. Since $3$ does not go into $0$ or $1$ we use zeros as placeholders.
	$0.12\div 3=0.04$

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[ohm_question]146600[/ohm_question]

Watch the following video to see another example of how to divide a decimal by a whole number.

Thumbnail for the embedded element "Example: Dividing a Decimal by a Whole Number"

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Most commonly, this calculation will be done with money while shopping. Prices of products tend to be presented as a combination of dollars and cents (i.e. $\$5.75$ , $2.99$ , or $\$25.60$ ). You can then divide by the number of items or the number of units to determine the cost per item or unit.

example

Suppose a case of $24$ water bottles cost $$3.99$ . To find the price per water bottle, we would divide $$3.99$ by $24$ , and round the answer to the nearest cent (hundredth).

Divide: $$3.99\div 24$
[reveal-answer q=”929706″]Show Answer[/reveal-answer]
[hidden-answer a=”929706″]

Solution

	$$3.99\div 24$
Place the decimal point in the quotient above the decimal point in the dividend.
Divide as usual. When do we stop? Since this division involves money, we round it to the nearest cent (hundredth). To do this, we must carry the division to the thousandths place.
Round to the nearest cent.	$$0.166\approx $0.17$
	$$3.99\div 24\approx $0.17$

This means the price per bottle is $17$ cents.

[/hidden-answer]

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[ohm_question]145993[/ohm_question]

[ohm_question]156951[/ohm_question]

Next, we will divide a whole number by a decimal.

example

Divide: $4\div 0.05$
[reveal-answer q=”616625″]Show Answer[/reveal-answer]
[hidden-answer a=”616625″]

Solution

	$4\div 0.05$
The signs are the same.	The quotient is positive.
Make the divisor a whole number by ‘moving’ the decimal point all the way to the right. Move the decimal point in the dividend the same number of places, adding zeros as needed.
Divide. Place the decimal point in the quotient above the decimal point in the dividend.
Write the quotient with the appropriate sign.	$4\div 0.05=80$