Arithmetic with Decimal Numbers

Arithmetic with Decimal Numbers

Watch this video lesson and see how our little professor goes about adding, subtracting, multiplying, and dividing decimal numbers all without the use of a calculator!

Decimal Numbers

Mr. Prof is our little professor for this video. He's about to show us how he adds, subtracts, multiplies and divides decimal numbers. He's picking up his chalk, and he's getting ready.

First order of business he wants to take care of is the difference between decimal numbers and regular numbers. Decimal numbers, he says, are numbers with a decimal point, while regular numbers are the numbers you normally use to count with. A decimal point is a dot used to show that a particular number has a part of it that is less than 1. For example, the number 1.5 means that we have a whole one plus half a one. The 0.5 is the half a one.

All of these are examples of decimal numbers because all of them have a decimal point:

1.5, 2.01, 0.7, 0.95, 3.14, 1.2349

Now that we've talked about what decimal numbers are, let's see how we go about adding and subtracting decimals. Watch how Mr. Prof does it.

Adding and Subtracting

Let's add 0.5 and 0.81 together and see what happens. Mr. Prof has just written these on his board. He's written the 0.5 on the first line and immediately under, he's written 0.81. But, something looks different. Instead of the 5 and the 1 lining up because they are the last digits, the 5 and the 8 are lined up.

Why is that? This is because when you are working with decimals, the decimal point matters. You want to line things up according to the decimal, like the 8 and the 5 here.

When adding and subtracting decimals, line the numbers up according to the decimal point.

Now that we have our problem correctly set up and written down, our next step is to drop that decimal point down into our answer area. So, we write a decimal point in line with the other two decimal points.

Once we have our decimal point where we want it, we will now go ahead and add these numbers the way we know how. I don't see a number above the 1, but since it's after the decimal point, I can go ahead and add a zero there. This is exactly what Mr. Prof has done. So, I add 0 plus 1 equals 1. So, I put a 1 underneath the 1 in the answer field.

Next, I have 5 plus 8 equals 13. Because 13 is greater than 9, I carry the 1 over and write a little 1 on top of the 0, and I write 3 underneath the 8 in my answer field. Next, I have 1 plus 0 plus 0, which equals 1. So, I write 1 underneath the 0 in my answer field. So my answer, as Mr. Prof also shows, is 1.31.

So, when adding decimal numbers together, what you have to remember is to line them up with the decimal point when you set up your problem. After that, you add the way you know how.

Now, what about subtracting decimal numbers? Yes, you guessed it, we subtract in a similar way - by setting up our problem with the decimal points lined up. Let's subtract 0.5 from 0.81 to see how it works. Since I am subtracting 0.5 from 0.81, I will write 0.81 on the top line and 0.5 on the second line, remembering to line them up according to the decimal point. Mr. Prof, I see, has already done that.

With everything lined up, I then write a decimal point in the answer field in line with the other decimal points. Then I go ahead and subtract the usual way. Since I am working on numbers after the decimal point, if I don't see a number, I can add a 0. So, I have 1 minus 0, which is 1. Then I have 8 minus 5, which is 3. Okay. So, my answer is 0.31.

Also, for subtracting, we remember to always subtract the smaller number from the larger. If the problem is asking us to subtract a larger number from a smaller number, then our answer will be negative. For example, if we were subtracting 0.81 from 0.5, then our answer is still 0.31, but negative 0.31. Mr. Prof gives us a high five for making it this far. Now, let's go on to multiplying.

Multiplying

Multiplying decimals is similar to multiplying our regular numbers with the exception of the last step, where we place our decimal point in our answer. Up until that point, we essentially ignore the decimal points. When we set up our problem for multiplication, instead of lining it up according to the decimal, we line it up according to the last