Subtraction strategies with hundredths

About some strategies subtracting decimals that involve hundreds.

So, for example, if I have 0.69 or 69 hundredths, and from that I want to subtract 0.34 or 34 hundredths, what is that going to be? Pause this video and see if you can compute this.

So, there are a bunch of ways to think about it. One way to think about it is this: this is 69 hundredths, and from that we are subtracting 34 hundredths. So this boils down to, I have 69 of something—in this case, hundreds—and I'm going to take away 34 of them. So what am I left with?

Well, what's 69 - 34? Well, 9 ones - 4 is going to be 5, and 60 - 30 is going to be 30. So I'm left with 35 hundredths, which I can write as 0.35.

Now, another way I could think about it is I could break up the tenths and then the leftover hundredths. I could view this as 610 - 310. So 610 - 310 plus 900 - 400s. We're going to get the same answer.

So, I have 610, and I take away 310. That's going to give me 310, or I could just write that as 0.3, and then to that, I would have to add 9 hundredths minus 4 hundredths, which is 5 hundredths. So, 0.05.

So, 310 and 5 hundredths is going to be 35 hundredths. Or we could just write it this way: we could write it as 310. We have a three in the ten place and 5 hundredths. Let me do that in blue color and 500s. Or we could do that as 35 hundredths.

However, these are different ways of thinking about subtracting these hundredths. Let’s do another example.

So, let’s say we want to compute, and this actually will probably be a little bit more straightforward: 3 and 43 hundredths minus 2. What is this going to be equal to? Pause the video and see if you can figure it out.

Well, it might jump out at you that this is the same thing as 3 + 43 hundredths minus 2. So, you can just look at the ones. You can look at the holes. We have three holes, and we're going to take away two of them. So we're going to be left with one hole, and we still have this 43 hundredths.

So, it's going to be one and 43 hundredths, or we could write that as 1.43. So, that one maybe was a little bit more straightforward.

But now, let's kind of combine the ideas of these last two examples into one that might seem a little bit more daunting.

So, let’s say that we want to subtract— we want to figure out what 65.79 minus 42.58 is. Pause the video and see if you can figure this out.

So, there are multiple ways to do this. You could separate the whole numbers. So you could say this is 65 - 42, plus and then think about the hundredths: 79 hundredths minus 58 hundredths.

And I’ll just rewrite this in words just to reinforce that this is 79 hundredths. You could say it's 7 tenths and 9 hundredths, but it's the same thing as 79 hundredths. This is 58 hundredths.

So, 65 - 42: 5 - 2 on the ones, we're going to get 3, and then 60 - 40 is equal to 20. So we have 23. Plus now, 79 hundredths minus 58 hundredths: 70 - 50 is 20, and 9 - 8 is 1.

So, this is going to be 21 hundredths, which we can write as 0.21.

So, when we compute this, it'll be 23 and 21 hundredths, and this is just one way to tackle it. There are multiple ways that you could try to tackle a subtraction problem like this.