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Current time:0:00Total duration:5:48

Strategies for subtracting basic decimals

CCSS.Math:

Video transcript

going to do in this video is begin to practice subtracting decimals and we're going to build up slowly and in future videos we're going to learn to do this faster and faster and doing it for more and more complex situations so let's say we have three tenths minus two tenths what is this going to be well there's a bunch of ways you could tackle it I encourage you to pause the video and try to do it on your own before I work through it with you but I'm assuming you you did that so let's do it together so you could view this as three tenths and then we're going to take away two tenths so if I have three of something and I take away two of them what am I going to be left with well I'm going to be left with one I'm going to be left with one tenth and we can visualize this let me put a hole there and this whole is divided into tenths and we see that three of the tents are already highlighted so these three green bars you can visualize is three tents now we want to take away two tenths so we want to take away we're going to take away one tenth and then we take away two tenths and so what are we left with well we're going to be left with this one tenth right over here that's the only tenth that is left of these three tens so now let's build on that idea and try to tackle more complex situations so let me deal let me delete this and this and let's say we want to tackle one point five or one and five tenths and from that we want to subtract zero point seven or seven tenths pause the video and see if you can figure this out so there's a couple of ways you could think about this and I'll tell you the way that I do it in my head you could view this as so let me rewrite this you could view this as one plus five tenths minus seven tenths minus let's see blue color - 7/10 and so there's a couple of ways that you could view it you could view this as ten tenths one hole is 10 10 so this is 10 tenths plus five tenths minus seven tenths minus seven tenths and so you could say this is 15 tenths if you're doing it in your head you might get to this faster you might say hey one point five one in five tenths is the same thing as 15 tenths minus seven tenths minus seven tenths so 15 of something minus seven of it well 15 minus seven is going to be eight so this is going to be equal to 8/10 so the way I just did I just thought of everything in terms of tenth something even one and five tenths you could view this as if you could somehow put a fifteen in the tenths place and instead call this zero ones and fifteen tenths and then you subtract seven tenths from those 15 tenths to get the eight tenths which would be 0.8 or 8/10 now another way that you could thought about it let's go back to this step right over here I could have said well look I can think about what one minus seven tenths is going to be so I could use as one minus seven tenths and then I'm going to add five tenths to that so then I'm going to add five tenths to that so one minus seven tenths one is ten tenths so if I take seven of them away I'm going to be left with three tenths so you could say that's going to be three tenths plus five tenths plus five tenths which is once again it is equal to eight tenths now another way that you could tackle this and once again I'm showing as many ways as possible just to appreciate but these are just different ways of tackling the same idea let's draw a number line here so let's say so let's say this is 0 and then 1 2 3 4 5 6 seven eight nine ten that is one one two three four five that is one point five so we're going to start at one point five right over here so that's one point five and we're going to take away seven tenths so one way that you might do it is you say okay let's we could take away we could take away five tenths which is a tickets to one and then we have to take two more tenths away which is going to take us to eight tenths or 0.8 so the way that I just thought about it just now is I said hey this is the same thing as 1 point 5 minus 5 tenths minus 5 tenths minus 2 tenths the reason why I broke it up like this it's like okay one point five or one in five tenths minus five tenths that's pretty straightforward to compute you could say that right over there is just going to be one I'm taking away the five tenths and then I have two tenths left over to take away so one minus these two tenths and once again you are left with eight tenths so these are all perfectly legitimate ways to tackle this problem and this is a way that many people including myself would try to do it in their head