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Estimating decimal addition

CCSS Math: 5.NBT.B.7

Video transcript

- [Instructor] What we're going to do in this video is get some practice estimating adding decimals. Here it says 12.93 plus 6.1 is approximately equal to, this little squiggly equal sign means approximately equal to, so try to estimate this and pick the right choice. Pause the video and try to do that. Alright, now let's work through this together. We could work this out by hand, but the whole goal here is estimation, so if I were doing this in my head I'd say, well, look, 12.93 is a little bit less than 13, and 6.1 is a little bit more than six. And it's actually nice because when I rounded I went down on this number and I went up on this number. I went up 700ths to get to 13 and I went down 1/10th to get to six, so I feel good when I add them up that we're going to get pretty close to the actual answer. 13 plus six is equal to 19, and lucky for us that that is one of the choices. There's multiple ways that you could try to estimate things. For example, you could do a little finer of an estimation. You'd say, this is approximately 12.9 plus 6.1, and then you'd say, 12 plus six is 18, and then, 9/10ths plus 1/10th is another one, so you'd say this is 18 plus one, and you'd say, okay, that, once again gets me pretty close to 19, or, this gets me to 19. Let's do another example. Here we're told five plus one plus 4.91 is approximately equal to what, so, pause this video and see if you can figure that out. Try to estimate it, try to do it without any paper. The way I would do it is actually very similar to the way we just did the last one. 5.1 is approximately equal to five, and, is approximately equal to five, and 4.91 is also approximately equal to five. I rounded to the nearest whole number in either case, and so, that would be equal to 10. Once again, it makes sense because we rounded down here and we rounded up here, we rounded down to go from 5.1 to five, and we rounded up to go from 4.91 to five. If you're rounding down in both cases, or rounding up in both cases your estimate might get a little bit further from your actual answer, so you might want to be a little bit careful. Alright, so in this last example, once again, pause the video and see if you can work through it on your own. We're just going to do the same thing here. This is going to be approximately equal to three plus, I rounded down, 3.14, rounded to the nearest whole number, three. 5.92 rounded to the nearest whole number is six. And once again, just like I mentioned in the previous ones, this is good because I am rounding down from 3.14 to three, I took away 1400ths, and I add 800ths to 5.92, so it's going to get me reasonably close to my actual answer, and so, three plus six is equal to nine, which is, indeed, one of the answers here. And this is actually interesting, you're not always going to see a multiple choice, so if you're trying to estimate things, but in this case, you know that just three plus five is going to be eight, and so this can't be the choice. Then you're going to have eight plus whatever 1400ths plus 9200ths is, and you know that's more than one, so you know our answer's going to be a little bit over nine. Once again, nine is a good approximation.