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Contributor: Briana Pincherri. Lesson ID: 11353
Uh oh! You have to multiply decimal numbers, but the decimal points don't line up! Don't panic. It's easy! Learn how to place decimals in your product!
Each of these was used to solve arithmetic problems before calculators were invented!
Drop that abacus and learn how to do it yourself with pen and paper!
When you multiply decimals, you may assume that you would line up the decimal points and bring them right down in the same location in your answer.
However, if you did this, you would be INCORRECT!
That logic works perfectly for adding and subtracting decimals, but it isn't needed when multiplying them.
Well, take a look!
When you are multiplying decimals, follow these steps.
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 ignore the decimal(s) and multiply like you know how.
Once you get to your answer, you must decide where the decimal should go.
To figure it out, 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 to the Got It? section to take a further look and complete some practice problems!
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