Easy Decimal Arithmetic: Count Them! (Multiplying)

Contributor: Briana Pincherri. Lesson ID: 11353

Uh oh! I have to multiply decimal numbers, and the decimal points don't line up! Don't panic; it's easy! Watch these videos and take some online practice to see how to place decimals in your product!

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:

How would you set up and solve the problem 1.23 x 45.6 (without a calculator!)?

When you multiply decimals, you may assume that you would just line up the decimal points and bring them right down in the same location in your answer.

However, if this is what you did, you would be INCORRECT! That logic works perfectly for adding and subtracting decimals, but it isn't needed when multiplying them. So, what do you do? Well, let's take a look!

When you are multiplying decimals:

  • Rewrite the problem vertically and multiply like normal (Just ignore the decimal points).
  • Count how many numbers were behind each decimal in the problem to start.
  • Make sure the product has the same amount of numbers behind the decimal as the sum of numbers behind decimals in the problem.

NOTE: When you rewrite the problem vertically (on top of each other), you should put the number with the most digits on top. REMEMBER, the decimals DO NOT have to line up!

Example:

2.2 x 1.12

      1 . 1 2 ← Number with more digits goes on top.
  x     2 . 2  
        2 2 4  
  +   2 2 4 0  
      2 4 6 4  
               
    2 . 4 6 4 Count how many numbers are behind the decimal in the problem. There are 3, so move the decimal over 3 places in the answer.

 

As you can see in the above example, when you rewrite the problem to multiply, you put the number with more digits (the longer number) on top. Then, you can just ignore the decimal(s) and multiply like you know how. Once you get to your answer, you have to decide where the decimal should go.

To figure it out, just count how many digits were behind the decimal in the original problem. You will move your decimal place over that many spots in your answer. DONE!

Continue on to the Got It? section to take a further look and see how you do with some practice problems!

Elephango's Philosophy

We help prepare learners for a future that cannot yet be defined. They must be ready for change, willing to learn and able to think critically. Elephango is designed to create lifelong learners who are ready for that rapidly changing future.