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Strategies for subtracting more complex decimals with tenths

Sal uses place value and decomposing numbers to subtract decimals to the tenths place, such as 2-1.2.

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  • leaf green style avatar for user Asim Yigit Uludag
    why do we line up decimal points when adding and subtracting?
    (13 votes)
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    • piceratops ultimate style avatar for user Ashaz Moen
      We line up decimal points and stuff a lot because then we have a critically low chance of mixing up places, like thinking the tenths are in the hundredth, getting the whole thing wrong, and having to restart frustrated. Sorry no one answered you for a long time. (Please comment and/or upvote this if it was helpful.)
      (2 votes)
  • female robot grace style avatar for user Nadia Coleman
    Why did he say 1 and 0 tenths is 0 and 10 tenths?
    (5 votes)
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  • blobby green style avatar for user nw71587
    my question is how can i do this the easy way?
    (3 votes)
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  • duskpin ultimate style avatar for user MirrorShards
    I have two questions:

    1) How do you create a time stamp?

    2) So, is the easiest way to subtract decimals stacking? Or is there a better way?
    (3 votes)
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    • hopper jumping style avatar for user Yuvraj M
      for you first question,
      I am not sure what you mean by creating a time stamp, but you could do the on a coding platform, I think.
      for your second question,
      There is no easy way and hard way. everyone has their own strengths and weaknesses. though many people say that stacking is a very efficient way of subtracting, other people say that there are other ways that they use that are very efficient. I think that stacking is very good in my opinion, but you might have a different one. so make sure that you find the one that you think is more efficient for you.
      (2 votes)
  • blobby green style avatar for user Gaby Spiegel
    Sal: Do some more examples subtracting decimals. So let's say we wanna figure out what two minus 1.2 is. Pause this video and see if you can calculate this. So there is multiple ways to tackle this. One way is you could say, "Look, this is the same thing "as two minus one "minus 2/10." This is two minus one and 2/10. So we're subtracting one, and we're subtracting 2/10. Now two minus one is pretty straightforward to compute. Two minus one is going to be one, and then we need to subtract 2/10 from that. So one is the same thing as 10/10. We could say this is 10/10. 10, write it this way. 10/10, and we're gonna subtract 2/10. What is that gonna give us? Well, that's going to give us, if we have 10/10, and we take away 2/10, that's going to give us 8/10. Lemme write it down here. 8/10, which is the same thing as 0.8, or 8/10. One way to think about this is if you're subtracting 0.2 from one, if you view this one as a 1.0, instead of expressing this as one, one, and 0/10, we are thinking about this as zero ones and 10/10. And when you think about it as zero ones and 10/10, well, 10/10 minus 2/10, it's easier to then think about, well, 10 of something minus two of something is gonna be eight of something. It's gonna be 8/10. Now we can also visualize this on a number line, so for example, let me draw a number line here, and so zero, one, two, three, four, five, six, seven, eight, nine. That's one, one, two, three, four, five, six, seven, eight, nine. That is two, so we're starting at two, and we're subtracting one and 2/10. So one way to think about it is we first subtracted one. We first subtracted one, and then we subtracted 2/10, and that got us to the point 8/10 on the number line. Another way to think about this, and the whole point here is to see multiple strategies and for you to think about what strategy you like the most and to realize they all get you to the same place. If you're thinking about it in a reasonable way, is you could use, what's the difference between two and 1.2? So 1.2 sits here on the number line, so what's the difference between two and one and 2/10? Another way to think about it is how many tenths do you have to add to one and 2/10 to get to two? Well, if you've already got 2/10, you need to add another 8/10 to get to the next whole. So you have to add 8/10, or you have to add 0.8. So the difference between two and 1.2, the difference between two and 1.2 is equal to 0.8.

    Me: do 10 - 2 and and a period in front of your anwser and boom your done with the first question.
    (3 votes)
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  • mr pants teal style avatar for user Jacob
    how do i do this its super hard
    (2 votes)
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  • duskpin seedling style avatar for user Lual Akol
    isnt doing the number line take more time then doing stacked up version?
    (2 votes)
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  • duskpin seedling style avatar for user Lual Akol
    Do some more examples subtracting decimals. So let's say we wanna figure out what two minus 1.2 is. Pause this video and see if you can calculate this. So there is multiple ways to tackle this. One way is you could say, "Look, this is the same thing "as two minus one "minus 2/10." This is two minus one and 2/10. So we're subtracting one, and we're subtracting 2/10. Now two minus one is pretty straightforward to compute. Two minus one is going to be one, and then we need to subtract 2/10 from that. So one is the same thing as 10/10. We could say this is 10/10. 10, write it this way. 10/10, and we're gonna subtract 2/10. What is that gonna give us? Well, that's going to give us, if we have 10/10, and we take away 2/10, that's going to give us 8/10. Lemme write it down here. 8/10, which is the same thing as 0.8, or 8/10. One way to think about this is if you're subtracting 0.2 from one, if you view this one as a 1.0, instead of expressing this as one, one, and 0/10, we are thinking about this as zero ones and 10/10. And when you think about it as zero ones and 10/10, well, 10/10 minus 2/10, it's easier to then think about, well, 10 of something minus two of something is gonna be eight of something. It's gonna be 8/10. Now we can also visualize this on a number line, so for example, let me draw a number line here, and so zero, one, two, three, four, five, six, seven, eight, nine. That's one, one, two, three, four, five, six, seven, eight, nine. That is two, so we're starting at two, and we're subtracting one and 2/10. So one way to think about it is we first subtracted one. We first subtracted one, and then we subtracted 2/10, and that got us to the point 8/10 on the number line. Another way to think about this, and the whole point here is to see multiple strategies and for you to think about what strategy you like the most and to realize they all get you to the same place. If you're thinking about it in a reasonable way, is you could use, what's the difference between two and 1.2? So 1.2 sits here on the number line, so what's the difference between two and one and 2/10? Another way to think about it is how many tenths do you have to add to one and 2/10 to get to two? Well, if you've already got 2/10, you need to add another 8/10 to get to the next whole. So you have to add 8/10, or you have to add 0.8. So the difference between two and 1.2, the difference between two and 1.2 is equal to 0.8.

    Me: do 10 - 2 and and a period in front of your anwser and boom your done with the first question.
    (2 votes)
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  • male robot johnny style avatar for user kaiden.lucian
    it is just like regrouping for subtraction so 1.00 - 0.2 or 0.20= 0.80
    (2 votes)
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  • starky tree style avatar for user Dannygreenlee
    Can decimals be in negative numbers?
    (2 votes)
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Video transcript

- [Instructor] Do some more examples subtracting decimals. So let's say we wanna figure out what two minus 1.2 is. Pause this video and see if you can calculate this. So there is multiple ways to tackle this. One way is you could say, "Look, this is the same thing "as two minus one "minus 2/10." This is two minus one and 2/10. So we're subtracting one, and we're subtracting 2/10. Now two minus one is pretty straightforward to compute. Two minus one is going to be one, and then we need to subtract 2/10 from that. So one is the same thing as 10/10. We could say this is 10/10. 10, write it this way. 10/10, and we're gonna subtract 2/10. What is that gonna give us? Well, that's going to give us, if we have 10/10, and we take away 2/10, that's going to give us 8/10. Lemme write it down here. 8/10, which is the same thing as 0.8, or 8/10. One way to think about this is if you're subtracting 0.2 from one, if you view this one as a 1.0, instead of expressing this as one, one, and 0/10, we are thinking about this as zero ones and 10/10. And when you think about it as zero ones and 10/10, well, 10/10 minus 2/10, it's easier to then think about, well, 10 of something minus two of something is gonna be eight of something. It's gonna be 8/10. Now we can also visualize this on a number line, so for example, let me draw a number line here, and so zero, one, two, three, four, five, six, seven, eight, nine. That's one, one, two, three, four, five, six, seven, eight, nine. That is two, so we're starting at two, and we're subtracting one and 2/10. So one way to think about it is we first subtracted one. We first subtracted one, and then we subtracted 2/10, and that got us to the point 8/10 on the number line. Another way to think about this, and the whole point here is to see multiple strategies and for you to think about what strategy you like the most and to realize they all get you to the same place. If you're thinking about it in a reasonable way, is you could use, what's the difference between two and 1.2? So 1.2 sits here on the number line, so what's the difference between two and one and 2/10? Another way to think about it is how many tenths do you have to add to one and 2/10 to get to two? Well, if you've already got 2/10, you need to add another 8/10 to get to the next whole. So you have to add 8/10, or you have to add 0.8. So the difference between two and 1.2, the difference between two and 1.2 is equal to 0.8. Let's do a few more examples that get a little bit more involved. So let's say we wanna calculate what 3.8 minus 1.5 is. Pause the video and see if you can calculate this. Well, just like before, we could view this as three minus one, so we're subtracting the ones plus 8/10 minus 5/10. Notice we have three and 8/10 minus one and 5/10, minus one and minus 5/10. And so now we can figure out, okay, three minus one, that's just going to be equal to two, and then 8/10 minus 5/10, well, that is 3/10, and so this is going to be two and 3/10, which of course we could write as 2.3. That seemed pretty straightforward. Let's do one that's a little bit more involved. Let's say we wanna calculate four and 5/10 or 4.5, minus two and 8/10 or 2.8. Pause the video and see if you can calculate this. So you might wanna do the exact same thing. You might say, "Okay, well, this is the same thing. "Let's think about the ones." This is four minus two plus 5/10 minus 8/10. Four and 5/10 minus two and minus 8/10, minus two and 8/10. That's exactly what we have up here. Then you would say, "Alright, four minus two, "that is two." But then you get to 5/10 minus 8/10, and there's multiple ways to tackle this, but you might say, "Oh, how do I take away 8/10 "if I've only got 5/10 here?" And there's a bunch of strategies that you could think about. You could say, "Hey, what if I can get "some more tenths here?" So this is 5/10 minus 8/10. What if I could get some more tenths here? The best way I can think of it is like what if I were to break up these ones because one is 10/10? So I could view a two as one plus one, or I could view this as one plus 10/10. So if you view a two as one and 10/10, you can then just add those. You could then figure out what is 10/10 plus 5/10 minus 8/10? What is that going to be? Well, this is a little bit more straightforward. 10/10 plus 5/10 is going to be 15/10, and if you have 15/10 and you take away eight of them, you're going to be left with 7/10. So this gets us to one plus 7/10, so all of this when you compute it, that is 7/10, which I could write as 0.7, which is equal to, so this is equal to one and 7/10. Now there's other strategies that you could do here. One strategy that, and this is the one that I typically do in my head is I write this as 4.5 minus 2.5 minus 0.3, or four and 5/10 minus two and 5/10 minus 3/10. Now why did I write it this way? Because I find this pretty straightforward to compute, and then once I get that answer, I just have to take away 3/10. So for example, well, if I have 5/10 here, and I'm taking away 5/10, those are gonna knock each other out, and so I'm just gonna be left with four minus two, which is gonna be two, and then I have to take away the 3/10. This is a pretty straightforward way of doing it in your head. So what's 3/10 less than two? Well, you can visualize a number line in your head. Well, that's gonna be one and 7/10, one and 7/10, and if this feels strange how I got, oh, one, I wrote 7/100, one and 7/10. And if this seems strange how I got there that fast, just think about this. This is the same thing as one plus one minus 3/10, and this one right over here is the same thing as 10/10, and so 10/10 minus 3/10. That over there is going to be, that is going to be 7/10, so it's going to be one and 7/10, which is what we got before. So the whole point here is to appreciate there's multiple strategies for subtracting decimals, some that you can do a little bit more automatically, but it's really good to think about what's going on in your head. And some strategies are actually better in your head than on paper, or at least more easy in your head.