Easy Decimal Arithmetic: Keep It! Count Them! Move It Up! (All Operations)

Contributor: Briana Pincherri. Lesson ID: 11355

"Rah! Rah! Rah! Go decimals!" You may not be that excited about decimal arithmetic, but there is a cheer (and an easy video) to help you remember the arithmetic rules. Online practice clinches it too!

categories

Middle School

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

Lesson Plan - Get It!

Audio:

Picture a cheerleader cheering to the crowds. What is her chant? "Keep it, count them, move it up!" What are they talking about? Is your team about to score and then need to keep, count, or move something? Take a minute to cheer it out loud for yourself, then jump into this lesson to be reminded exactly what it stands for!

You aren't at a football game getting ready to watch your team score, but you will be ready to score some points adding, subtracting, multiplying, and dividing decimals correctly.

Let's get started and review the arithmetic rules for decimals!

Adding and subtracting decimals

  • Key first step: Make sure your decimals are lined up (rewrite all problems vertically, up and down!).
  • Add or subtract as normal.
  • KEEP the decimal right where it was and drop it down into your 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:

8.2 + 2.11

Rewrite, and line up the decimals:

      8 . 2 0 ← Added 0 here.
  +   2 . 1 1  
    1 0 . 3 1 Add like normal, and drop the decimal down.

 

2.2 - .14

Rewrite, and line up the decimals:

      2 . 2 0  
  -   0 . 1 4 ← Added 0 in front.
      2 . 0 6 Subtract normally, and drop the decimal down.

 

Multiplying decimals

  • Rewrite and multiply as normal (ignore the decimal points).
  • Count how many numbers were behind each decimal in the problem to start.
  • Use that total to count over from the right that many places in the answe, and you've got it!

NOTE: When you rewrite the problem vertically (on top of each other), you can always put the "longer" number on top. The decimals DO NOT have to line up!

Example:

1.2 x 3.25

      3 . 2 5 ← Number with more digits goes on top.
  x     1 . 2  
        6 5 0  
    + 3 2 5    
      3 9 0 0  
               
    3 . 9 0 0 Count how many numbers are behind the decimal in the problem. There are 3, so move the decimal over 3 places in the answer.

 

Dividing decimals

If the decimal is in the first number (dividend):

  • Divide like normal, as if the decimal isn't there.
  • At the end, bring the decimal straight up from where it is so it is in that same place in the answer.

If the decimal is in the second number (divisor):

  • First, shift the decimal in the dividend until you have a whole number.
  • However many places you moved it in the divisor, you also have to move it that many places in the same direction in the dividend.
  • Then, divide as normal, bringing the decimal straight up into the answer!

The example for division will be in the next section. It is a good one to see, so make sure you pay close attention, and don't be afraid to watch it more than once. Go to the Got It? section to watch it now.

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