Video: Adding Two Decimal Numbers

Calculate 154.502 + 29.699.

03:56

Video Transcript

Calculate 154.502 plus 29.699.

This calculation asks us to add two decimal numbers together. Both numbers have three decimal places. So there are lots of digits to add here. To make sure we add these numbers correctly, we’re going to have to use a written method. And we can use column addition. To begin with, we can write our first number, 154.502. Now, we write our second number underneath, 29.699. Is this how to write it? We’ve written our second number in the wrong place. The digit two in 29 has a value of two tens. And yet, we’ve written it in the hundreds place. How can we make sure that both numbers are lined up correctly?

Well, when we’re adding two decimals like this, the best thing to do is to draw our decimal point to start with. And we draw it directly underneath the first decimal point. So the two are lined up. Now, we know that each of our digits is going to be in the correct place, 29.699. Now, if we check, our two tens are level with the five tens of 154. Each digit is in the correct column.

But before we start, there’s another mistake that we need to avoid. It would be very easy to work through the calculation from right to left to add each column and to think that that’s how to find the answer. But if that’s all we do, then our answer is going to be wrong. We need to also include a decimal point. Our answer is going to be a decimal number. It’s so easy to forget that decimal point. So before we start, let’s draw a decimal point within our equal sign. And again, we’re going to line it up with the other two decimal points above it. And now, we can add each column from right to left.

Two thousandths plus nine thousandths equals 11 thousandths. We can exchange 10 of these thousandths for one hundredth. Zero hundredths plus nine hundredths is nine hundredths. But we’ve also got the one hundredth that we’ve just exchanged. That takes us to 10 hundredths, which is the same as one tenth. Five tenths plus six tenths equals 11 tenths. And if we include the one tenth that we’ve exchanged, that takes us to 12 tenths. We can exchange 10 of our 12 tenths for one whole.

Now, we’re into the whole number part of our calculation. Four plus nine equals 13 plus the one that we exchanged underneath gives us a total of 14. And 14 is the same as one ten and four ones. In the tens column, five tens plus two tens equals seven tens, including the 10 underneath. That gives us a total of eight tens. And finally, we can write this digit in the tens place. And we don’t need to exchange. We had to exchange in all the other columns. In the hundreds column, we only have one hundred to add.

In this problem, we added two decimal numbers using column addition. And we thought carefully because we wanted to avoid making two mistakes. Firstly, we made sure that we lined up the decimal points first. This way, we knew that all of the digits were in the correct columns. And then secondly, we drew a decimal point in between our equal sign so that we didn’t forget it at the end.

The total of 154.502 and 29.699 is 184.201.

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