Estimating adding decimals

What we're going to do in this video is get some practice estimating adding decimals. So here it says twelve point nine three plus six point one is approximately equal to this little squiggly equal sign means approximately equal to. So try to estimate this and pick the right choice.

So pause the video and try to do that.

All right, now let's work through this together. So we could work this out by hand, but the whole goal here is estimation. If I were doing this in my head, I'd say, well, look, twelve point nine three is a little bit less than thirteen, and six point one 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. So I went up seven hundredths to get to thirteen, and I went down a tenth to get to six. So I feel good when I add them up that we're gonna get pretty close to the actual answer.

So, thirteen plus six is equal to nineteen, and lucky for us that that is one of the choices.

There are multiple ways that you could try to estimate things. For example, you could do a little finer estimation. You'd say this is approximately twelve point nine plus six point one, and then you say twelve plus six is eighteen.

And the nine tenths plus one tenth is another one, so you'd say this is eighteen plus one, and you'd say, okay, that once again gets me pretty close to nineteen, or this gets me to nineteen.

Let's do another example. So here we're told five plus one plus four point nine one 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. Well, the way I would do it is actually very similar to the way we just did the last one. Five point one is approximately equal to five, and four point nine one is also approximately equal to five.

I rounded to the nearest whole number in either case, and so that would be equal to ten. It makes sense because we rounded down here, and we rounded up here. We rounded down to go from five point one to five, and we rounded up to go from four point nine one 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.

All right, so in this last example, once again pause the video and see if you can work through it on your own.

Well, we're just going to do the same thing here. This is going to be approximately equal to three plus, I rounded down, three point one four rounded to the nearest whole number is three. Five point nine to round it to the nearest whole number is six.

And once again, just like I mentioned in the previous ones, this is good because I'm rounding down from three point one four to three. I took away fourteen hundredths, and then I added eight hundredths to five point nine two. So it's going to get me reasonably close to the 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 gonna 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, and then you're gonna have eight plus whatever fourteen hundredths plus ninety-two hundredths is, and you know that's more than one. So you know our answer is going to be a little bit over nine.

So once again, nine is a good approximation.