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Current time:0:00Total duration:3:29

Estimating adding and subtracting 3-digit numbers

Video transcript

what we're going to do in this video is get some practice estimating adding and subtracting three-digit numbers and so here it says 398 + 251 is and this squiggly equal sign means we just need to make an estimate we want to know what is this roughly equal to so normally I say pause the video but here I want you to work it out in your head so I'm just gonna pause I'm just gonna stop talking for like a second and see if you can come up with what this is or maybe I'll give you 3 seconds because the whole point here is to estimate not get the exact right answer okay so I'm assuming you've had a go at it now I'll tell you how I think about it we of course could maybe take out some paper or you might even be able to get the exact answer in your head but in everyday life if you're at the supermarket and you're like okay yeah I guess it would be an expensive supermarket something is $398 something else is 251 dollars roughly how much would I be spending if I bought both of them well my head would say alright 398 I would round to nearest 10 at least maybe the nearest hundred so let's round to the nearest 10 and in the case of 398 the nearest 10 is the nearest hundred and so we would get to this is approximately 400 and then this is 251 is approximately 250 so if you were to add these two things up you're going to get something that is approximately equal to 650 and lucky for us that is one of the choices and what I'm about to do is not estimate but just see how far off we were so if we were to just actually do it 398 + 251 8 plus 1 is 9 9 plus 5 is 14 this is 14 10 so we put 4 tens here and the other 10 tens we regroup as a hundred and then one plus three plus 200 is 600 so in this case in this particular case we got awfully close with our estimation and if you don't need to get the exact number well that that'll that'll be fine let's do another example and here we are going to do a little bit of subtraction so once again pause this video or we're trying to do it as quickly as you can try to estimate what this difference is going to be sick 178 minus 273 alright let's do it together so there's a couple of ways to tackle it you could round to the nearest ten and you could say well this is approximately 600 let's see if you round to the nearest ten you would round up right over here so you'd say this is 680 - you could say 270 which is going to give you and here you're still gonna have to do some mental math this would be what 600 minus 200 is 480 minus 70 is 10 so that's one way to do it or you could do it even rougher you could just do to the nearest hundred so you could say look 678 the nearest hundred is 700 and then minus 273 the nearest hundred there's 300 and so this is even rougher but it gets us actually still similar answers and either way we estimated either the kind of rougher way or the slightly less rough way we got to something around 400 so you could feel good that this is your choice the reason why we even have to make multiple choice questions for estimation is not everyone is going to get the exact same estimate but the correct choice should be the one that a wreath at any reasonable estimate would be closest to