Easy Decimal Arithmetic: Keep It! (Adding and Subtracting)

Contributor: Briana Pincherri. Lesson ID: 11352

What's the point of learning about decimals? The point is, the point is all-important! Where you place it makes the difference between right and wrong answers. Videos and online practice show you how!

categories

Middle School

subject
Math
learning style
Visual
personality style
Lion, Otter
Grade Level
Middle School (6-8)
Lesson Type
Quick Query

Lesson Plan - Get It!

Audio:

If you have $23.15 and then find another $2 in your couch, how much money do you have? This is easy addition, right? Well, guess what? You are not only adding in this problem, but using an important math rule as well. Can you figure out what it is?

When you are adding money, regardless how big or small the amounts, you are not only using your addition skills, but also using a rule with decimals, whether you know it or not!

Let's take a look at the example mentioned above, where you have $23.15 and find $2 more. What total amount of money do you have? You probably already have said $25.15. Easy, right?

Well what if you added it like this:

    $ 2 3 . 1 5
  +         $ 2
               

 

If I write it this way and add with the 2 directly under the 5, our answer would be $23.17, and would mean that you only found two cents. You want to show that you found two whole dollars, which is $25.15.

Now do you see what you need to do in order to answer the problem correctly? Tell your parent or teacher how to set up the problem so you can show that you found two dollars.

Did you explain the rule you need to use?

When adding decimals, you must follow the rule that you must LINE UP the decimal places before you add or subtract. From the example, $2 is the same as $2.00. If I line up my decimals correctly, this problem looks like this:

    $ 2 3 . 1 5
  + $   2 . 0 0
    $ 2 5 . 1 5

 

Chances are you did that problem in your head, so you didn't really have to worry about lining up the decimals. Your brain is used to working with money, so it is more than likely automatically putting decimals in the right places. The good news is that the same rules apply to ALL DECIMALS, not just money. That means, if you can answer questions that involve adding and subtracting money, you can easily add and subtract ANY decimal.

Let's take a look at the rules for adding and subtracting decimals:

Adding and subtracting decimals

  • Write the problem vertically.
  • Line up your decimals, even if you need to add zeros as in the example above.
  • Add or subtract as normal.
  • Bring the decimal down to the same place in the answer.
  • You are done!

NOTE: If you have a problem where there isn't a number showing in a certain spot, you can simply put a 0 there for it. But remember, make sure your decimals are lined up!

Examples:

10.2 + 4.06

Rewrite and line up the decimals:

    1 0 . 2 0 ← Added a 0 here.
  +   4 . 0 6  
    1 4 . 2 6 Add as usual and bring down the decimal.

 

8.86 - .25

Rewrite and line up the decimals:

      8 . 8 6  
  -   0 . 2 5 ← Added a 0 in front.
      8 . 6 1 Subtract normally and bring down the decimal.

 

Seem easy enough? Head to the next section to see just how well you've got it down.

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