Main content

## 6th grade

# Adding decimals: 9.087+15.31

CCSS.Math:

Sal solves 9.087+15.31 using the "standard algorithm". Created by Sal Khan.

## Video transcript

Let's see if we can
add 9.087 to 15.31. And I encourage you
to pause the video and try to do it on your own. So I'm assuming you have
tried to do it on your own. And now let's see how we
could actually tackle this. Now, one thing I want to
point out, some of you all might have seen
these numbers all lined up and immediately want
to say, hey, 7 plus 1 is 8, and 8 plus 3 is 11, carry
the 1, et cetera, et cetera. And if you did that, you
would be making a mistake. Because, you see,
right over here, these decimals aren't lined up. Here, if you did that, you would
be adding the 7 thousandths to 1 hundredths. You would be adding
0 tenths to 5 ones. You would be adding 9 to 1
tens, or essentially, this is a 10 right over here. So the places would
be all mixed up. So what you need to do is to
actually align the decimals so that your place
values are aligned. So what you want to do is
you want to align things up. So we could write 9.087. And then we want to
align the decimal. So let's align the decimal. This is what has to match up. And this is going to be 15.31. And this should hopefully
make sense to you as well. This is 9 point something
plus 15 point something so it's going to be--
If you add 9 to 15, it will be 24 point something,
give or take a little bit. And you see that here. Here you have a 9 plus the 15. So you have lined up the
appropriate place values. And now we are ready to add. It's a good idea to start
with the smallest place value, so if you have any extra
at a certain place, you can bring something
into the next place value. So here you say 7 plus--
well, this is 7 thousandths. It's in the thousandths place. And you might want to-- you
say, well, what do I add it to? There are no thousandths
right over here. And you're right. There are no thousandths. So we could literally
write 0 thousandths. So 7 thousandths plus 0
thousandths is 7 thousandths. 8 hundredths plus 1
hundredth is 9 hundredths. 0 plus 0 tenths plus
3 tenths is 3 tenths. We got our decimal. Then you have 9 ones
plus 5 ones is 14 ones. Well, 14 ones is the same
thing as 4 ones and 1 ten. So we'll carry that
1 right over there. This is just 1 ten plus
4 ones, which is 14. And so then finally, you have
1 ten plus another ten is 2. So we get 24.397.