Basic Algebra and Geometry Made a Bit Easier Lesson Plans:
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Lesson 69: Multiplying Decimals
If you are in high school or college, it is likely that you are allowed to use a calculator on every exam. Still, it is important to have a general idea of how basic arithmetic works. In this lesson we'll look at how to multiply decimals.
First, review all of the previous lessons on decimals, and make sure that you fully understand them. Decimals were covered over many previous lessons. Multiplying decimals is easy, as long as you follow a few rules.
Let's say we want to multiply 17.3 + 28.47. First, do not add any zeroes to the right of any number like we did when adding. Leave the numbers as they are, and just line them up on the right. Do not line up the decimal points like we did when adding. The problem is set up below:
28.47
x 17.3
Multiply the numbers like you've learned before, but completely ignoring the decimal points. Pretend that they are not there at all. If it helps you, rewrite the problem with the decimal points completely removed so that you aren't confused with your columns.
After multiplying as usual, place a decimal point to the right of the answer, and then move it to the left one space for each decimal space in the numbers that you multiplied. For example, in these numbers, we had two decimal places in the first number, and one decimal place in the second number. That means we move the decimal point 3 spaces to the left in our answer, and then that becomes our answer.
In the problem above, when we multiply we get 492531. We have to move the decimal point three spaces to the left, so our answer becomes 492.531.
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