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CCSS Math: 5.NBT.B.7

Video transcript

- [Instructor] Alright, now let's get some practice estimating, subtracting decimals. So, over here it says 12.93 minus 6.1 is approximately equal to what? This squiggly-looking equal sign you can view as roughly equal to or approximately equal to. So, pause this video and see if you can figure it out. And, you shouldn't really need paper for it. The whole point of estimation is to be able to do it quickly and get close to the answer. You don't have to get the exact answer. Well, let's think about it. The way my brain would do it is 12.93-- Let's just round each of them to the nearest whole number. So, this is going to be approximately 12.93 if I round to the nearest one or to the nearest whole number, I'm gonna round up to 13. And 6.1, if I round to the nearest whole number, I'm gonna round down to six. So, I could say this is roughly 13 minus six which is equal to seven, and that is indeed one of the choices there. And, when you're doing estimation, you might have gotten a slightly different result, but I'm guessing that the way that this question was set up, that if you do any type of reasonable estimate, that you're going to have a number that is closer to seven than any of the other choices. And to be clear, sometimes you could do this in your head. For example, 12.93 minus 6.1, you could say 12 minus six is going to be six. And then you have 93 hundredths minus, you could do this as 10 hundredths. So, it's gonna be 93 hundredths minus 10 hundredths is 83 hundredths, plus .83. So, just like that, you could do something like this and say, alright, the answer here is gonna be 6.83 exactly. And that, once again, is approximately equal to seven. But, the whole point of this exercise is to get comfortable estimating things because a lot of times in life you don't need the exact answer. You just need a rough answer, just to know the ball park of what you're talking about. Let's do another example. So here, we are asked what is 56.75 minus 46.9 approximately equal to? Once again, pause this video and try to work it out. Well, here I'm gonna do the same thing. I'm just gonna round each to the nearest whole number and then subtract. And, I'm gonna write it out, but you could do this in your head. So 56.75 rounded to the nearest whole number is 57 round up, minus 46.9, I would also round up to 47. And, in our head this is pretty straightforward. This is going to be equal to ten. And, that is indeed is one of the choices. And, normally in life when you're estimating, you don't have choices like this. I guess this is just the easiest way for-- This if off of the Khan Academy exercises. So, for someone to be able to grade it because estimations can be different, so you have to pick the one that's at least closest to what you came up as your estimate. Now, let's do one more example. And, I will write in purple. Alright. So, pause the video again. See if you can figure this out. Alright, so 49.7, I'm gonna round up to 50, 4.16 I'm gonna round down to four. So, that's going to be approximately equal to 30-- Oh sorry, that's gonna be approximately equal to 46. And, we do indeed see 46 as one of the choices right over there. So, hopefully you feel comfortable. There's nothing fancy going on here. The whole point of this is to just get comfortable doing things like this in your head and estimating and rounding numbers so you can get a ball park sense of what this difference is going to be.