About this video
Sal subtracts 0.794 from 39.1. After lining up the decimals and filling empty places with 0, he regroups 1 one into 10 tenths (making 11 tenths), then passes tenths to hundredths and hundredths to thousandths, and subtracts to get 38.306.
You'll be able to
- line up decimals and fill empty places with zeros to subtract.
- regroup across several places, from ones to tenths to hundredths to thousandths.
- subtract a decimal with more decimal places from one with fewer.
Key words
- regroup
- Trade 1 of a bigger place for 10 of the next smaller place so you can subtract.
- place value
- The value of a digit based on its position, like ones, tenths, hundredths or thousandths.
Transcript
Let's try to calculate 39.1 minus 0.794, and so pause the video and try this on your own. All right, I'm assuming you've given a go at it, so now let's work through it together. So I'm going to rewrite this. It's 39.1 minus-- I'm going to line up the decimals so that I have the right place values below the right place values-- minus-- this 0 is in the ones place, so I'll put it in the ones place-- 0.794.
And now we're ready to subtract. Now, how do we subtract 4 from nothingness here, and 9 from nothingness here? Well, the same thing as nothing is a 0. And so now we can start to think about how to subtract. Well, we still have the problem. Well, we're trying to subtract 4 from 0, so we're trying to subtract 9 from 0.
So what we could do is take this one tenth and try to regroup it into the hundredths place and the thousandths place. So let's think about this. If we make this-- actually that's not actually going to solve our problem. Well we could do it, but then we're going to have zero tenths, and we're still going to have a problem here.
So actually let me go to the ones place. So let me get rid of a ones, so that's eight ones, which is going to be 10 tenths. So that's going to now-- we're going to have 11 tenths. The 10 tenths from here plus 1 is 11 tenths. Now let's take one of those tenths so that we have 10 tenths, and give it to the hundredths.
So that's going to be 10 hundredths. And now let's take one of those hundredths-- so now we have nine hundredths-- and give it to the thousandths. So that's going to be 10 thousandths. Now we're ready to subtract. So 10-- let me do this in yellow-- 10 minus 4 is 6.
9 minus 9 is 0. 10 minus 7 is 3. We have our decimal point. 8 minus 0 is 8. And then we have 3 minus nothing is 3. So we're done, 38.306.
Video © Khan Academy, used under CC BY-NC-SA. Watch on Khan Academy. Puzzicle is not affiliated with Khan Academy.