How to Multiply With Decimal Points

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Multiplying with decimal points is the same as any other multiplication problem, as long as the amount of decimal points for each number is noted. Move the decimal left for the number of decimals in a multiplication equation with help from a math teacher in this free video on using decimals in math.

Part of the Video Series: Fractions & Proportions
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Video Transcript

So, how does one multiply with decimal points? Hi. I'm Jimmy and I've been teaching college math for nine years now. And when you multiply with decimal points, it's actually your run of the mill multiplication problem. There's one condition you have to think about and that's how many decimal points each number has, but you're actually going to get to that until the very end. But until then, it's actually your standard multiplication problem. So, here's a basic example for you. Suppose you wanted to multiply 5.2 by .31. Now, unlike addition or subtraction, where you have to line up the decimals exactly, with multiplication, you don't worry about that. Because .31 has two numbers, you just put it where, if you don't have to line them up, as long as the 1 is with the 2 and the 3 is with the 5. You'll worry about the decimal points later. So here is where the multiplication starts. Just like you would at the 1, multiply with the two numbers up top. So, 1 times 2 is 2. 1 times 5 is 5. Don't worry about the decimals until the very end. For the second line, remember that when you get to the next line, always put a zero or leave it blank. It's up to you. And then, 3 times 2 is 6. And then 3 times 5 is 15, where you put the 15 right over here. And, as you know, with the standard multiplication, you just add all the lines that you have here. And again, just add normally. 2 and 0 is two. 5 and 6 is 11. Carry the 1. 1 and 5 is 6. And then, the 1 is there, so you have a total of 1,6,1 and 2. Now, here's where you have to take into account the decimal points. The top number is to the one decimal place. The bottom number is a two decimal point number. So, what you have to do is to figure out exactly where your decimal lies, just count the total number of decimal places. One, two, three. So, from here, just move the decimal left the number of places. One, two, three. And they end up there. So, that means 1.612 is actually what you get when you take 5.2 times .31. So, I'm Jimmy, and that's an example as to how you multiply with decimal points.


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