Decimals

Multiplying with decimals

In Lesson 6, you learned how to add decimal numbers. You may be able to think of times when you'd add decimals in real life. For example, let's say you go to the store and find a shirt you really like. The price tag says it costs $15.60. You like the shirt so much that you decide to buy five of them.

To figure out the total cost, you could add the prices.

Adding this many numbers could take a long time. In the lesson on multiplication, we learned that when you multiply, you are increasing a number many times. Because all of the shirt prices are the same, multiplication could help you solve this problem a little faster.

When you multiply decimal numbers, it's helpful to set up the problem in a way that makes it easier for you to solve it one step at a time.

Click through the slideshow below to learn how to set up a multiplication problem with decimals.

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  Instead of adding $15.60 + $15.60 + $15.60 + $15.60 + $15.60...

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  Instead of adding $15.60 + $15.60 + $15.60 + $15.60 + $15.60...we'll multiply $15.60 by 5.

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  Let's set up our multiplication expression: $15.60 x 5. We'll stack the numbers one on top of the other.

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  It's a good habit to place the number that has the most digits on top. This makes the problem easier to solve.

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  Let's look at the number of digits in each number. 15.60 has four digits...

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  Let's look at the number of digits in each number. 15.60 has four digits...and 5 is one digit.

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  15.60 has more digits. This means we'll write 15.60 above the 5.

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  Since we're multiplying this number, we'll write the times sign (X) to the left of the numbers.

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  Instead of an equals sign (=), we'll put a line underneath the number on bottom.

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  When writing a stacked multiplication expression with decimal numbers, the numbers should be lined up on the right.

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  Let's look at another example. We'll stack this expression: 4.5 x 38.12.

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  First, let's look to see how many digits are in each number. 4.5 has two digits...

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  First, look to see how many digits are in each number. 4.5 has two digits...and 38.12 has four digits. 

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  38.12 has more digits. This means we'll place 38.12 above 4.5.

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  Then we'll make sure the digits to the right are lined up. The 2 is right above the 5.

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Now you try it. Set up these multiplication expressions. Don't solve them yet—just set them up.

At a bookstore, you buy three novels. Each novel costs $7.99.

The cost of your child's school lunch is $3.75 per day. You give your child enough money to buy lunch for 5 days.

You fill your car's tank with 8 gallons of gas. Each gallon costs $3.68.