To multiply two-digit decimals, we can use the usual method of removing and replacing the decimal point.

As an example, let's learn how to compute 0.4 \times 0.9. We proceed as follows:

Step 1. First, ignore the decimal points and multiply as if both numbers were whole numbers: 4 \times 9 = 36

Step 2. Then, insert the decimal point in the result so that number of decimal places is equal to the total number of decimal places in both factors combined.

Here, there is \color{blue}1 decimal place in 0.4 and there is \color{blue}1 decimal place in 0.9, so their product will have a total of {\color{blue}1} + {\color{blue}1} = 2 decimal places.

We take our value of 36 and insert a decimal point to make a number with 2 decimal places:

0\,\overset{\color{red}\downarrow}{\color{red}\bbox[2px, lightgray]{.}}\!\!\!\underbrace{36}_{\large\text{[math]2[/math] digits}}

Therefore, 0.4 \times 0.9 = 0.36 \, .

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What is 0.2 multiplied by 0.4?

EXPLANATION

First, we ignore the decimal point and multiply as if both numbers were whole numbers:

2 \times 4 = 8

We now count the total number of decimal places in the two factors.

There is \color{blue}1 decimal place in 0.2 and there is \color{blue}1 decimal place in 0.4, so their product will have a total of {\color{blue}1} + {\color{blue}1} = 2 decimal places.

We take our value of 8 and insert a decimal point to make a number with 2 decimal places:

0\,\overset{\color{red}\downarrow}{\color{red}\bbox[2px, lightgray]{.}}\!\!\!\underbrace{08}_{\large\text{[math]2[/math] digits}}

Therefore, 0.2 \times 0.4 = 0.08 \, .

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$0.3 \times 0.7 =$

a
$0.021$
b
$0.01$
c
$0.21$
d
$0.1$
e
$2.1$

$0.8 \times 0.7 =$

a
$0.06$
b
$5.6$
c
$0.056$
d
$0.56$
e
$0.6$

We can use the same method to multiply decimals even when they are greater than 1. For example, to compute

4.7 \times 0.8,

we first ignore the decimal points and multiply as if both numbers were whole numbers. We ignore any leading {\color{red}{0}} 's during the multiplication:

\begin{align*} & \begin{array}{ccccc} & & \!\!\!\!\! \substack{ \\ \color{blue}5}{} \!\!\!\! & \\ & & \!\!\!\! 4 \!\!\!\! & \!\!\!\!\!\!\! . 7 \!\!\!\! \\ \!\!\!\!\times\!\!\!\! & & \!\!\!\! \color{red}0 \!\!\!\! & \!\!\!\!\!\!\! . {8} \!\!\!\! \\ \hline & \!\!\!\! 3 \!\!\!\! & \!\!\!\! 7 \!\!\!\! & \!\!\!\! 6 \!\!\!\! \end{array} \end{align*}

We now count the total number of decimal places in the two factors.

There is \color{blue}1 decimal place in 4.7 and there is \color{blue}1 decimal place in 0.8, so their product will have {\color{blue}1} + {\color{blue}1} = 2 decimal places.

We take our value of 376 and insert a decimal point to make a number with 2 decimal places:

3\,\overset{\color{red}\downarrow}{\color{red}\bbox[2px, lightgray]{.}}\!\!\!\underbrace{76}_{\large\text{[math]2[/math] digits}}

Therefore, 4.7 \times 0.8 = 3.76 \, .

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What is 0.8 \times 9.4?

EXPLANATION

First, we ignore the decimal points and multiply as if both numbers were whole numbers:

\begin{align*} \require{cancel} %%%%%%%%%% %%% Step A %%% %%%%%%%%%% & \begin{array}{ccccc} & & & \!\!\!\!\!\! \color{lightgray} \substack{ \fbox{[math]\color{blue}7[/math]} \\[2pt] \fbox{[math]\color{blue}3[/math]} } \!\!\!\! & \\ & & & \!\!\!\! 0 \!\!\!\! & \!\!\!\!\!\!\! . 8 \!\!\!\! \\ \!\!\!\!\times\!\!\!\! & \!\!\!\!\phantom{0}\!\!\!\! & \!\!\!\!\phantom{0}\!\!\!\! & \!\!\!\! 9 \!\!\!\! & \!\!\!\!\!\!\! . 4 \!\!\!\! \\ \hline & & \!\!\!\! \!\!\!\! & \!\!\!\! 3 \!\!\!\! & \!\!\!\! 2 \!\!\!\! \\ \!\!\!\!+\!\!\!\! & \!\!\!\! \!\!\!\! & \!\!\!\! 7 \!\!\!\! & \!\!\!\! 2 \!\!\!\! & \!\!\!\! 0 \!\!\!\! \\ \hline & \!\!\!\! \!\!\!\! & \!\!\!\! 7 \!\!\!\! & \!\!\!\! 5 \!\!\!\! & \!\!\!\! 2 \!\!\!\! \\ \end{array} \end{align*}

We now count the total number of decimal places in the two factors.

There is \color{blue}1 decimal place in 0.8 and there is \color{blue}1 decimal place in 9.4, so their product will have {\color{blue}{1}} + {\color{blue}{1}} = 2 decimal places.

We take our value of 752 and insert a decimal point to make a number with 2 decimal places:

7\,\overset{\color{red}\downarrow}{\color{red}\bbox[2px, lightgray]{.}}\!\!\!\underbrace{52}_{\large\text{[math]2[/math] digits}}

Therefore, 0.8 \times 9.4 = 7.52 \, .

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$1.4 \times 0.9 =$

a
$12.6$
b
$1.26$
c
$1.49$
d
$9.36$
e
$14.9$

$0.7 \times 3.5 =$

a
$2.27$
b
$15.4$
c
$2.45$
d
$24.5$
e
$2.75$

What is the value of 1.1 \times 4.5?

EXPLANATION

First, we ignore the decimal points and multiply as if both numbers were whole numbers:

\begin{align*} \require{cancel} %%%%%%%%%% %%% Step A %%% %%%%%%%%%% & \begin{array}{ccccc} & & & \!\!\!\! 1 \!\!\!\! & \!\!\!\!\!\!\! . 1 \!\!\!\! \\ \!\!\!\!\times\!\!\!\! & \!\!\!\!\phantom{0}\!\!\!\! & \!\!\!\!\phantom{0}\!\!\!\! & \!\!\!\! 4 \!\!\!\! & \!\!\!\!\!\!\! . 5 \!\!\!\! \\ \hline & & \!\!\!\! \!\!\!\! & \!\!\!\! 5 \!\!\!\! & \!\!\!\! 5 \!\!\!\! \\ \!\!\!\!+\!\!\!\! & \!\!\!\! \!\!\!\! & \!\!\!\! 4 \!\!\!\! & \!\!\!\! 4 \!\!\!\! & \!\!\!\! 0 \!\!\!\! \\ \hline & \!\!\!\! \!\!\!\! & \!\!\!\! 4 \!\!\!\! & \!\!\!\! 9 \!\!\!\! & \!\!\!\! 5 \!\!\!\! \\ \end{array} \end{align*}

We now count the total number of decimal places in the two factors.

There is \color{blue}1 decimal place in 1.1 and there is \color{blue}1 decimal place in 4.5, so their product will have {\color{blue}{1}} + {\color{blue}{1}} = 2 decimal places.

We take our value of 495 and insert a decimal point to make a number with 2 decimal places:

4\,\overset{\color{red}\downarrow}{\color{red}\bbox[2px, lightgray]{.}}\!\!\!\underbrace{95}_{\large\text{[math]2[/math] digits}}

Therefore, 1.1 \times 4.5 = 4.95 \, .

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$5.4 \times 1.3 =$

a
$70.2$
b
$0.702$
c
$6.72$
d
$67.2$
e
$7.02$

$4.3 \times 8.7 =$

a
$37.41$
b
$36.14$
c
$38.14$
d
$3.471$
e
$37.91$
Example: Word Problems

The circuit of a bicycle speed track is 0.60 miles long. How many miles will a competitor have covered by completing 8.5 laps on the track?

EXPLANATION

First, notice that 0.60 = 0.6.

To find out the number of miles a competitor covers by completing 8.5 laps on the track, we need to multiply 0.6 by 8.5.

First, we ignore the decimal points and multiply as if both numbers were whole numbers:

\begin{align*} \require{cancel} %%%%%%%%%% %%% Step A %%% %%%%%%%%%% & \begin{array}{ccccc} & & & \!\!\!\!\!\! \color{lightgray} \substack{ \fbox{[math]\color{blue}4[/math]} \\[2pt] \fbox{[math]\color{blue}3[/math]} } \!\!\!\! & \\ & & & \!\!\!\! 0 \!\!\!\! & \!\!\!\!\!\!\! . 6 \!\!\!\! \\ \!\!\!\!\times\!\!\!\! & \!\!\!\!\phantom{0}\!\!\!\! & \!\!\!\!\phantom{0}\!\!\!\! & \!\!\!\! 8 \!\!\!\! & \!\!\!\!\!\!\! . 5 \!\!\!\! \\ \hline & & \!\!\!\! \!\!\!\! & \!\!\!\! 3 \!\!\!\! & \!\!\!\! 0 \!\!\!\! \\ \!\!\!\!+\!\!\!\! & \!\!\!\! \!\!\!\! & \!\!\!\! 4 \!\!\!\! & \!\!\!\! 8 \!\!\!\! & \!\!\!\! 0 \!\!\!\! \\ \hline & \!\!\!\! \!\!\!\! & \!\!\!\! 5 \!\!\!\! & \!\!\!\! 1 \!\!\!\! & \!\!\!\! 0 \!\!\!\! \\ \end{array} \end{align*}

We now count the total number of decimal places in the two factors.

There is \color{blue}1 decimal place in 0.6 and there is \color{blue}1 decimal place in 8.5, so their product will have {\color{blue}{1}} + {\color{blue}{1}} = 2 decimal places.

We take our value of 510 and insert a decimal point to make a number with 2 decimal places:

5\,\overset{\color{red}\downarrow}{\color{red}\bbox[2px, lightgray]{.}}\!\!\!\underbrace{10}_{\large\text{[math]2[/math] digits}}

Therefore, 0.6 \times 8.5 = 5.10 \, .

We conclude that a competitor will have covered 5.1 miles when they complete 8.5 laps.

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Practice: Word Problems

Tara needs to buy $1.4$ ounces of flaked almonds for a cake recipe. If each ounce of flaked almonds costs $\$1.60,$ how much will she have to pay?

a
$\$2.24$
b
$\$2.44$
c
$\$2.14$
d
$\$2.34$
e
$\$2.54$
Practice: Word Problems

Lenny's horse takes one second to ride $7.2$ meters. How far will it run in $8.5$ seconds?

a
$63.2$ meters
b
$57$ meters
c
$62.2$ meters
d
$61.2$ meters
e
$60.2$ meters
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