Subtracting decimals (tenths and hundredths)
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- [Sal] Let's say we wanted to compute eight and 38 hundredths minus four and 54 hundredths. See if you can pause this video, and figure it out on your own. There are multiple strategies for doing this. I'll tell you the way that my head likes to do this. I would view this as the same thing as eight and 38 hundredths, and then I would subtract four and 38 hundredths. The reason why I pick 38 hundredths is that will cancel out, it'll take away these 38 hundredths. And then I still have to subtract a little bit more. If I also subtracted 38 hundredths, I have to subtract a total of 54 hundredths. Let's see, 54 is how much more than 38? I am doing this in my head as I speak. To go from 38 to 48, I need six, I need 10 more. I need to subtract another 16 hundredths. How did I think about that? 54 is 38 and 16. If I'm subtracting 54, I could break that up. I could subtract 38 hundredths, and then I could subtract 16 hundredths. Why did I pick 38 hundredths? Because it matches up with this. And so, if I say eight minus four is going to be four, and then I have 38 hundredths minus 38 hundredths, those are just gonna cancel out with each other. So this minus that is just gonna be four. And then I'd say four minus 16 hundredths. So how do we think about that? Well, you might be able to do this in your head at this point. You might say: "OK, I have to go 16 hundredths "less than four, that's gonna be three and 84 hundredths." Or, you might wanna just break it up. You could say: "OK, four is the same thing as "three plus 100 hundredths." Hundredths. And then I wanna take away minus 16 hundredths. 100 minus 16 is 84. This part right over here is 84 hundredths. I'll write it out. Hundredths. Which I can write as 0.84. Three plus 0.84, that's going to be, this is going to be equal to, we figured it out already multiple times, three and 84 hundredths. Now, as I said, there's multiple ways that we can try to tackle this. For example, another way that I could do this one right over here is I could say, all right, lemme do the ones. I'd say, eight minus four. Eight minus four. Plus. Let me try to do the tenths. So, I could have three tenths minus five tenths. 0.3 minus 0.5. And then, I would have the hundredths. So, eight hundredths minus four hundredths. Plus eight hundredths minus four hundredths. Now, this is pretty easy to compute, this is pretty easy to compute, but what about this? I don't have five tenths to take away. What I could do is, instead of having an eight here, I could take one away from it, and make that a seven. And so, I have one that I can regroup someplace over here. I can reorganize it. So, I could view this as seven plus 10 tenths. And so, I can take those 10 tenths, I could take those 10 tenths, and add 'em right over here. Lemme rewrite this. I could rewrite this as seven minus four, plus 10 tenths, plus three tenths. Actually, lemme just write it all as plus three tenths. I'm mixing different ways of writing it this way, but that might be good. Minus five tenths. And then this part I could just compute. This is plus four hundredths. Zero point zero. Color. Plus four hundredths. This right here is gonna be four hundredths. Now what is this going to be? Seven minus four is going to be three. 10 tenths plus three tenths. The right color. This is going to be three. And then, 10 tenths plus three tenths is 13 tenths, minus five tenths is eight tenths, and then I have four hundredths. Once again, 3.84. Three ones, eight tenths, four hundredths. Or, three and 84 hundredths.