Does this look like a good estimate? 720 - 30 = 500

Does this look like a good estimate? 575 - 159 = 415

What is an estimated answer here? 604 - 350 =

What is this number rounded to the nearest 10? 996

Applying rounding for estimating (subtracting)

Key Stage 2

Maths

Addition and subtraction

Which of numbers does not need to be rounded to the nearest 10?

﻿Hello there, my name's Miss Brinkworth, I'm going to be going through this maths lesson with you today, which is all about rounding and estimating, both of which are super, super important skills to help you throughout your maths learning. So, find somewhere as quiet as possible and let's get started. So, our learning objectives today is that we're going to be applying rounding, so estimating, and we're going to be using that to help us with some subtraction questions. So, let's get started by looking at our lesson agenda. So, what we're going to start with is recapping multiples of 10, I'm sure a lot of you feel very comfortable with your 10 times table. We're going to look at them in a bit of detail, as that's going to really help with today's learning. We're going to practise our rounding to the nearest 10 and we're going to use that to help us estimate subtraction questions. Then, you'll have that chance for the independent work, just to embed that learning and give you a chance to have a go on your own and then the exit quiz, for you have a chance to see how well all that's gone in. So, all you need for today's lesson is a pen or pencil, some paper and that great attitude. So, please pause the video here and go and get those things. Wonderful, here is a little warm up for you then, some nice, simple subtraction questions, all of which are about multiples of 10, so a nice little recap on our multiples of 10 and our 10 times table. As we're going through them, think carefully about the strategies that you're using, what known facts you're already using and have a go at that challenge as well. Pause the video here for as long as you need. Wonderful, let's have a look at those answers then. So, 40 take away 30 is 10. We know that because we know that four take away three is one, this answer is just 10 times bigger than that, so 40 is 10 times bigger than four, 30 is 10 times bigger than three and so our answer is 10 times bigger than one. And that's the strategy that we can use to answer all of these questions. That's why most people find their 10 times tables quite easy really because we can just scale up by 10 from our ones. So, eight take away seven is one, 80 take away 70 is 10. There's an interesting one there with 30 take away 30, that will be really helpful with today's lesson, but just to remember that sometimes the answer is zero. So, 30 takeaway 30, if I have 30 pens and I give them all away, I've got none left. Well done if you managed to have a go at that challenge as well, where you needed to subtract two multiples of 10 to get these answers. Here's some ways in which you can do those but there's lots of lots of different ways and well done if you had a go at that challenge. Okay, so let's have a look here. How would we go about subtracting these larger multiples of 10 where we've got some three digit multiples of 10, like 150? Well, if we look at that first question, 150 take away 20, we can use the same strategy that we've used earlier in our warmup. So, 150 takeaway 20, I can change into 15 takeaway two. I can change them both down from multiples of 10 and I can make that a simpler sum. So, 15 takeaway two is 13, 150 takeaway 20 is 130 and I can use that on others of these questions as well. Obviously, some of them require some regrouping, so 300 take away 70, what I might want to think about is that if I'm taking away 70 away from a hundred, it's similar to seven away from 10, which gives me three, so I know that I'm going to have 30 there in my tens column and here's some other answers there for you as well. Can you see any question there that requires a little bit of different working out? Question two there, 210 take away 60 requires a little bit more working out and what I did to work that one out is I ignored the 10 to begin with, so I did 200 take away 50, so that gave me 150 and then because it's 60, that's 10 more, 210 is 10 more than 200, so I know that I could do 200 take away 50, would be the same as 210 take away 60. So, it's really useful to have those number facts at hand to mentally subtract these numbers. For that last one as well, it was a little bit more complicated in that we did have to be regroup a little. So, 50 is bigger than 40, but 10 bigger, so I know that if I'd taken away 40, I'd have got to 200, 10 is, sorry, 50 is 10 bigger than 40, so I needed to take 10 more away, so rather than just going back to 200, I went back 10 more to 190. Here's some for you to have a go at, I've given you a little bit of help on the first one, so pause the video here and have a go at subtracting these multiples of 10. Shall we see how you got on? Hopefully that clue helped you there a little bit. Ooh, I've made a little bit of a mistake there, haven't I? 11 take away three isn't 80, 11 take away three is eight but hopefully you could use that to help you with that question of 110 take away 30 is 80. And here's your other answers as well. Really well done if you got all of those right but do just have a look at them if you made some mistakes as this is what we're going to be doing with today's lesson. And really well done if you had to go at that last question where there was some regrouping needed. Okay, so what today's lesson is about is estimating, now estimating is a really, really useful skill and it's one that mathematicians use all the time. Well, what is it a really good guess, that's not a random guess, it's a very educated mathematical guess and it's as close to the most accurate answers we can get. And it's really useful for us to use when we're doing maths quickly, when we're just trying to get a good idea, when we're checking our answers, it can come in handy for a whole number of things. And as you move through school and continue with your math studies, you will round and estimate more and more often, so it's a really good skill to get used to now. So, what are we talking about when we talk about rounding to the nearest 10? Well, what we're doing, if we take this number 91, for example, we want to think about the two multiples of 10 that 91 is between, so 91 is in between 90 and 100. Those are both multiples of 10 and 91 is sitting between them. We then think about which one it sits closer to, and I've put 92 there, but 91 with the in between 90 and 92 and we can see that it is closer to 90. Your turn then, think about 87, I'll be nice and give you the two multiples of 10 that it sits in between, pause the video and have a think about whether you think when we round 87 to the nearest 10, does it go to 80 or 90? Which one do you think it's closer to? How did you get on? Well, this is where 87 sits along that number line, which has been split into 10 because there are 10 numbers between 80 and 90, 10 numbers between every multiple of 10. So, we can sit it along that number line and hopefully you can see that 87 is closer to 90 than it is to 80. So, when we round 87 to the nearest 10, we get 90. What about with a larger number then, like a three digit number? Well, we're still looking at the 10s column and we're still looking at which two multiples of 10 it sits in between. So, 764, it's bigger than 760 but it's not as big as 770, it sits in between those two numbers. We then just need to think about which one it's closest to. Well, 64 is closer to 60 than it is to 70 and there's a rule that we can remember and it will come up in a slide in a moment, but it's the rule that if it's one, two, three or four it goes down. So, we can see that this 64 has gone down to 60 instead of going up to 70 but don't get confused, when we say going down, it doesn't go down into the 50s, it goes down a 10 rather than going up a 10. So, it's gone down to 60 rather than going up to 70. Okay, here's another number for you, think carefully about which two multiples of 10 it fits in between before you try rounding it, pause the video here and have a go. Okay, which two multiples of 10 does 199 sit in between? Well, hopefully you can see that it's larger than 190 and it's smaller than 200. Well done if you got that, sometimes people do get confused when we're having to bridge onto the next 100. So, we know that anything in the 90s is between 90 and 100, or in this case, 190 and 200. And 199 is right there, really close to 200, so when we round 199, you will certainly be rounding it to 200 rather than down to 90. And here's that rule for us that we talked about, one, two, three or four in the one's column goes down, whereas five, six, seven, eight or nine goes up to the next 10. Okay, here's some for you to have a go at, pause the video and let me know what these numbers are rounded to the nearest 10. Okay, how did you get on? Sometimes people get a little bit confused with five, so we've got five in our ones column there for 55, but they round up when it's five. So, 55 goes to 60, 89, hopefully you can see that's very close to 90, as 90 is the next number after 89. 218 is between 210 and 220, it's closer to 220, well done for seeing that, and 501, it's only gone over 500 by one, so we round it down to 500. Really well done if you got all of those right. Okay, why are we rounding? Well, we're rounding today to estimate. So, here's a problem that might be a little bit tricky for us to do in our heads and it might be easier for us to have an estimate and it will be easier for us to round them before we estimate them, that's because when we round them, we'll turn them into multiples of 10. And we can see from our warm up and at the beginning of the lesson, that multiples of 10 are easy to subtract. So, here we might want to round both of them, we could round 97 to 100 and we could round 36 to 40 and the sum suddenly becomes much easier to answer. 100 take away 40, we can do that in our heads really quickly, we know that 10 take away four is six, so 100 take away 40 is 60. So, we can do that if we round both numbers but if we do round both numbers, we might be moving quite far away from the accurate answer and we might only need round one of them. So, we might just want to around 36 to 40, we've then got a question which is 97 take away 40 and that's not too hard to answer at all. We know we only need to amend the tens column, nine take away four is five, so 97 take away 40 is 57. Or we might just want to round 97 to 100, we've then got 100 take away 36 and that gives us 54. If we look at the accurate answer, the accurate answer is 61, so you can see that some are closer than others in that answer there. So, have a go and think about which one you would like to round to answer this question, pause the video and have a go. Okay, how did you get on? Well, here's both of them, well, here's one of them rounded, sorry. 59 take away 20 will give you 39 or if you rounded 59 to 60, you do 60 takeaway 23 is 37. The right answer is 36. Now, it might come down to personal preference which of those you find easier to round but you can see that if we round 59 to 60, we've only had to move 59 by one number to go from 59 to 60. So, our answer there will be quite accurate, it will only be one number off, whereas with 23, we've moved it three to get from 23 to 20. But it might be that we find 59 takeaway 20 an easier sum to do in our heads than 60 take away 23. So, some of this comes down to what you prefer and what you feel comfortable with because the point of rounding and estimating is that you can do it quickly in your head. It's a skill that allows you to quickly do some sums, to check your answers, to get an idea of what the answer might be. Okay, let's have a think about a three digit number then and rounding these to estimate. So, if we just rounded 197 to 200, we'd be looking at the question, 357 take away 200. I think that's quite an easy sum for us to do in our heads and I like if we can round to a multiple of 100, that makes things even easier than rounding to a multiple of 10, I think. 357 take away 200 is 157, if we rounded the other way, if we did 357 rounded to 360, we've then got 360 take away 197. I think that's quite a difficult sum to do in your head, so with this question, I think the best thing to do is to look and see that 197 is very close to 200 and that makes it a much easier sum to do in our heads. So some of it comes down to looking at the numbers carefully and thinking about which rounding is going to make that question easier for you to answer. And there's the answer there, the accurate answer, to see how close we can get just by rounding and doing those answers quickly in our heads. Okay, you're turn then, pause the video and decide which of these numbers you'd like to round to estimate the answer to this question. How did you get on? Which one did you decided to round, I wonder? Well, these were your options, you could round 531 to 530, you've then got 530 take away nine, not that difficult, maybe not the easiest, you have to do a little bit of regrouping, you have to do 30 take away nine. Or you could do 531 take away 110 you actually get exactly the same answer because we've just moved them each by one. So, it really, as you can see, can come down to personal preference. I think, I prefer 531 take away 110 because we can just amend the tens column but if you prefer the other one, that's absolutely fine. And the accurate answer is 422, so you can see that by rounding and estimating, we can get really close to the right answer. Okay, time for your independent work, pause the video, take as long as you need and come back for those answers in a moment. Okay, let's see how you got on. So, you just need to round these numbers to the nearest 10 and here are your answers, I'm really hoping everybody got the majority of those right, well done and which one is the odd one out? Hopefully you could see that 680 was already a multiple of 10 and didn't need rounding. This question then, some of my answers will be different to yours as we will have chosen to round differently. As long as your answer's are near mine, that's absolutely fine, remember that we're estimating in this lesson, so it's not about getting exactly the right answer, it's about rounding and estimating. And here are my answers but yours might not look like this, as long as they're near these answers, then that is absolutely fine. In terms of the challenge, it's a bit of a tricky one because really both of these comments are fine. It would be fine to round either of those numbers and both of them would give you a really good estimate to that question, so either of those would be fine. Right, time for the final knowledge quiz, you've done lots of work today with some new skills, so please do have a go at that quiz and see how much has gone in and enjoy the rest of your learning today, bye-bye.

Applying rounding for estimating (subtracting)

Applying rounding for estimating (subtracting)

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UnitMaths / Addition and subtraction

Maths