Video: Adding Two Decimal Numbers

Calculate the following: 10.2079 + 48.4128.

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Video Transcript

Calculate the following, 10.2079 plus 48.4128.

The two numbers in this addition are interesting because they’re fairly long decimal numbers. Each number contains two digits to start with. They’re the tens and the ones. And then, there are four digits after the decimal point. That means we go as far as ten thousandths.

Before we add the two numbers together, we can have a quick estimate of what we think the answer is going to be. We can see that the whole number part of these numbers adds up to 58. 10 plus 48 equals 58. And then, it looks like the decimal part of these numbers is not going to add up to another one. So, it looks like our answer is going to be 58 point something.

Let’s use the column method to find the answer and to see if our prediction is correct. First, we’ll write the addition out. And we’ll set it out vertically. And the best way to do this and to make sure that the digits are in the correct columns is to make sure that the decimal points are lined up. Once we’ve done that, we can fill in the digits around the decimal points. And we know that each set of digits has the correct value.

Finally, before we start adding, let’s put another decimal point in between the equal sign. This way we know we definitely won’t forget it. As usual, we’re going to work from right to left. So, we’re going to begin here with the ten thousandths column nine ten thousandths plus eight ten thousandths equals 17 ten thousandths. On to the thousandths column, seven thousandths plus two thousandths equals nine thousandths, plus the one we’ve exchanged equals ten thousandths, which is the same as one hundredth.

We’ve only got one hundredth in the top part of the addition, but we’ve also exchanged one, so we have two hundredths altogether. And two tenths plus four tenths equals six tenths. Notice how we didn’t have to exchange any ones there, so it looks like our prediction is going to be correct. The answer is going to be 58 point something.

Zero ones plus eight ones equals eight ones. And one 10 plus four tens equals five tens. Because there wasn’t a clear mental method to find the answer, we decided to use the column method. We wrote the two decimals out on top of each other and then we added the columns from right to left, remembering to put the decimal point in our answer. 10.2079 plus 48.4128 has a total of 58.6207.

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