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Subtraction strategies with hundredths

CCSS Math: 5.NBT.B.7

Video transcript

- [Instructor] About some strategies subtracting decimals that involve hundredths. For example, if I have 0.69, or 69/100, and from that I want to subtract 0.34, or 34/100, what is that going to be? Pause this video and see if you can compute this. There's a bunch of ways to think about it. One way to think about it is this is 69/100. Hundredths. And from that we are subtracting 34/100. Hundredths. And so this boils down to, I have 69 of something, in this case hundredths, in this case hundredths, and I'm gonna take away 34 of them. So what am I left with? Well what's 69 minus 34? Well nine ones minus, or I should say nine minus four is gonna be five. And 60 minus 30 is going to be 30. So I'm left with 35/100. 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. So I could view this as 6/10, let me circle that a little bit better, 6/10 minus 3/10. So 6/10 minus 3/10 plus... Plus 9/100 minus 4/100. So plus 9/100 minus 4/100. And we're gonna get the same answer, so that is 6/10 and I take away three of them. That's gonna give me 3/10, or I could just write that as 0.3. And then two that I would have to add 9/100 minus 4/100 is 5/100. So 0.05. So 3/10 and 5/100 is going to be 35/100, or we could just write it this way. We could write it as 3/10. We have a three in the tenth's place. And 5/100. Let me do that blue color. And 5/100. Or we could view that as 35/100. However way, these are different ways of thinking about subtracting these hundredths. Let's do another example. Let's say we want to compute. We would like to compute, and this actually will probably be a little bit more straightforward, three and 43/100, minus two. 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 three plus 43/100, minus two. And so you can just look at the ones. You could look at the wholes. We have three wholes, and we're gonna take away two of them. So we're gonna be left with one whole, and we still have this 43/100. So we still have this 43/100, so it's gonna be one and 43/100, or we could write that as 1.43. That one maybe was a little bit more straightforward. Now let's kind of combine the ideas of these last two examples, and do one that might seem a little bit more daunting. 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. Multiple ways to do this. You could separate the whole numbers, so you could say this is 65 minus 42. 65 minus 42, plus, and then think about the, and then think about the hundredths. Plus 79/100, so 79/100, minus 58/100. Minus 58/100. And I just, I'll rewrite this in words just to reinforce. This is 79 of something, 79/100. You could say it's 7/10 and 9/100, but it's the same thing as 79/100. Hundredths. This is 58/100. Hundredths. And so 65 minus 42. Five minus two go in the ones. We're gonna get three. And then 60 minus 40 is equal to 20. So we have 23 plus... Now 79/100 minus 58/100, 70 minus 50 is 20. And nine minus eight is one. So this is gonna be 21/100, which we can write as 0.21, 21/100. And so when we compute this it'll be 23 and 21/100. This is just one way to tackle it. There are multiple ways that you could try to tackle a subtraction problem like this.