Easy Decimal Adding and Subtracting: Keep It!

Lesson Plan - Get It!

Audio:

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

Look at the above example, 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? But what if you added it like this?

If you write it this way and add the 2 directly under the 5, your answer would be $23.17, meaning that you only found two cents.

You want to show that you found two whole dollars, which is $25.15.

• Do you see what you need to do to answer the problem correctly?
• How would you 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 LINE UP the decimal points before you add or subtract.

From the example, $2 is the same as $2.00. If you line up the decimals correctly, the problem looks like this.

Chances are you did that problem in your head, so you didn't 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.

Take a look at the rules for adding and subtracting decimals.

Adding and Subtracting Decimals

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

NOTE: If you have a problem with a number showing in a certain spot, you can 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.

8.86 - .25

Rewrite and line up the decimals.

• Seem easy enough?

Head to the Got It? section to see how well you've got it down.