Adding and subtracting tens from a 2-digit number

Key Stage 1

Maths

Addition and subtraction of 2-digit numbers

If I know 6 - 5 = 1. Then I know 60 - 50 = ____

If I know 7 - 3 = 4. Then I know 70 - 30 = ____

﻿Hello everybody and welcome to today's session. My name is Ms. Hughes and in today's lesson, we're going to be looking at adding and subtracting tens to two digit numbers. So let's get started. This is what our lesson agenda for today is going to look like. So we're going to start off by using known facts to help us with some addition, then you're going to have a talk task. Next, we are going to use known facts for subtraction, and finally, you will have a main task and of course your quiz to complete at the end of the video. So for today's lesson, you are going to need a pencil and rubber and some paper. So pause the video now to go and grab these things if you have not got them already. Brilliant, let's make a start then. So to begin our lesson today, I want to warm our brains up with these equations. We're going to try and solve these equations this morning using known facts. So I want you to look at all of these different equations and think about the number bonds within 10, that you know, that's all going to help you to answer these questions. Off you go. Let's go through these answers then. So looking at the first set of equations. If I've got 20 add 30, I know that I can use my number bond, two add three equals five to help me solve that equation. 30 add 60, I can use the equation three add six equals nine. 50 add 40, I can use the number bond five add four equals nine. 40 take away 20, I can use the number bond four take away two equals two. And these are the answers that you should have got. 20 add 30 equals 50. 30 add 60 equals 90. 50 add 40 equals 90 and 40 take away 20 equals 20. Let's look at these next ones now where we're adding three, two digit numbers. So what known facts could we've used to help us there? Well, I know that two add four add three equals nine. That was going to help me with this top one. So my answer that was 90. Three add one add six equals 10 was helping me to find out that all of these two digit numbers made 100. Three add to add four equals nine. That was going to help me work out 30 add 20 add 40 equals 90. And finally nine take away four, we know is five take away two equals three. That was going to help us to find 30. We're going to move on now team to think about using our known facts to help us with addition problems like this one on the board. So let's have a read of it. 56 people are on the train. 20 more people arrived. How many people are on the train now? To look at this problem or to solve this problem. I'm going to use a part whole model. So let's have a look at that. So we know 56 people are on the train. 20 more people arrived and we're looking to find out, how many people are on the train now? When we are looking at a word problem like this, we need to think about a few questions. Firstly, I need to think, What do I already know from this question? In other words, what values do I already know? Do I know the values of either of my parts? Do I know the value of my whole? Then I need to figure out what's missing. So what do I need to figure out? What part from my part whole model do I need to figure out? So from this question, I know that I'm looking to see how many people are on a train all together. I know that at the beginning there were 56 people and then 20 more people were added to that. So I know I've got the value of my two parts. My first part that I have is 56. So I've put 56 in my first part. And the second part that I have is 20. So I've got 20 in the second part of my part whole model. I'm trying to find the whole. So to do that, I need to add my two parts together. We can put both of my parts into my whole now, 'cause I'm adding them together and that will give us what our whole is. So let's count, 10, 20, 30, 40, 50, 60, 70, 71 72, 73, 74, 75, 76. So my whole is 76. We know that 76 people in total are on the train. Let's look at another way to solve this. So I could have used some of my number bonds within 100 to help me, sorry, within 10, to help me solve this problem. So I could have started with partitioning the number 56. I partitioned the number 56 this way, because I want my tens, my five tens, to be near to my two tens that I'm adding. So this is why I partitioned at this way round. So I've partitioned my 56 into five tens, which is 50 and six ones, which has six. Now we can look at our tens because I've got five tens and I'm adding two tens, which is 20. I know that five tens add two tens is seven tens, which has a value of 70. Right now I'm left with seven tens, which is 70 and six ones, which is 76. So some number facts help me to solve this. I knew that five add two is equal to seven. Therefore I was able to find out that 50 add 20 is equal to 70. Let's see what this would have looked like as a part whole model. So I have my 56 with its five tens and six ones. And I have my 20 with it's two ones. So what I was doing was adding my tens together. So my two tens, which equal 20 and my five tens, which equal 50, I added together first. So that gave me seven tens, which I knew is 70. And then I added my six ones at the end. There we go. And in total I'm left with 76. So 56 add 20 is equal to 76. Let's have a look at another question. This time it's not a word problem, but I've been given an equation which is 36 add 20. Let's have a look at how we could solve that in a part whole model. So I can see that the values that I do have are my parts and what I'm looking for is my whole. I know that one part is worth 36. So I've got 36 in the first part there. And the second part is 20. So I've put that in there. To work out my whole, I need to add these two parts together. So let's see what that looks like added together. There we go. So I've added my two parts together and they're now in the whole. So let's count them in tens and ones. 10, 20, 30, 40, 50, 51, 52, 53, 54, 55, 56. So altogether there are 56 people on the train. Let's look at another way to solve this. I could have used some number facts within 100 to help me figure this out. So we can partition 36 into three tens which is 30 and six ones, which is six. Now that I've partitioned my 36, I can see that I've got three tens and I'm adding two tens because I'm adding 20. So we can add those tens together. If I know that two tens add three tens will give me five tens. Then I know that 20 add 30 is 50. So now that I've added my tens and that equals 50, I just have my ones left over that I need to add on. So 50 add six is 56. Let's go do that one more time. If I know that three add two is equal to five, then 30 ad 20 is equal to 50. I was able to use that number bond within 10 to help me work out my tens added together. Let's have a look at this on a part whole model. So remember I have my two parts, 36 and 20, and I'm adding them together. I'm going to start by adding my three tens and two tens. My three tens from 36 and my two tens from 20. Like that. So I'm left with five tens, which is 50. And then I need to add my six ones. So in my whole I have 56. 36 add 20 is equal to 56. Alright, team, it's now time for your talk task. So you are going to do exactly as we've just done, looking at number facts to help us solve addition problems. So you're going to look at these equations that are on the board and you are going to think of or derive the number facts within 10 are going to help you work out your problem. I would like you to use these pink sentences at the bottom of the screen as well to explain what's happening. So we're going to look at this equation first, and then you are going to have a go at the rest yourself. I want to use these number sentences as I'm working out my equation. So I'm going to start with my equation, 31 plus 40, and I'm going to put my equal sign, but I don't know yet what that whole is. I only know the value of my two parts. I can see that I've got three here which represents 30 and four here represents 40. Three tens and four tens. So I can use the equation. Three ones add four ones is equal to seven ones. That helps me to know that three tens add four tens is equal to seven tens. So I know therefore that 30, which is three tens, add 40, which is four tens is equal to 70 Like that. Remember we've still got our one leftover so I need to add that on. So 70 add one gives me 71. So my total is 71. So you can see how I use my number bond three add four equals seven. To help me work out that answer. Pause it here now to have a go at your talk task and then play the video when you're ready to continue. We're now going to look at how we can use our number bonds or our known facts to help us subtraction for this half of the lesson. So let's have a look at a question on the board. 46 people are on the train. 20 people got off the train. How many people are now on the train? Let's look at this in a part whole model. Remember when we're doing a word problem, we need to think carefully to ourselves. What do we already know? And what are we trying to find out? So do I know the values of any of my parts here? Or do I know the value of my whole? and what part am I my trying to find out? Well, I can see from this question that 46 people were already on the train. So 46 people is the value of our whole. So let's put 46 in the whole. Brilliant. 20 is one of my parts because 20 people got off the train. So I'm going to take that away from my whole and put it into one of my parts, which would look like this. There we go. Now I'm left with 10 20, 21, 22, 23, 24, 25, 26. So 26 is my other part, which would go in here. So we know that 46 take away 20 leaves us with 26 left. So there are 26 people on the train. Let's have a look at how else we could have solved this using our number facts. So we could have partitioned our number 46 into tens and ones. So 46 has made up of four tens, which is 40 and six ones, which is six. So we now have four tens and I can take away two tens, which is 20. So 40 take away 20 is 20. Now I'm left with two tens and I have six ones left over. So we need to add those together so I'm left with 26. So let's look at those equations that helped us to figure that out. If I know that four take away two is equal to two, then 40 take away 20 is equal to 20. Let's have a look at this in a part whole model. So remember we started off with 46 as our whole. So there's 46 in the whole. There were 46 people on that train. Then we took away two tens. Now I'm left with two tens and then six ones on the end. So 46 take away 20 is equal to 26. Let's have a look at another example. This time, it's just a number equation. So we have 36 take away 20. Remember our whole is 36. So we've started with 36 and I can see that one of my parts is 20. So I'm going to put 20 in to that part. So I'm left with 10, 11, 12, 13, 14, 15, 16. So this part that was missing is 16, which means that 36, our whole take away 20 leaves us with 16. Let's look at how we could have solved it using our number facts. So 36, we could have partitioned into three tens, which is 30 and six ones, which is six. Then we could do our three tens, which is 30 take away 20, which is 10. So now I'm left with one 10 and six ones, which is 16. So if I know that three take away two is equal to one, then I know that 30 take away 20 is equal to 10. Let's look at that on our part whole model again. So we've got 36 has our whole, we're taking away one part, which is 20. So I've taken away my two tens and I'm left with one 10 and six ones. So 36 take away 20 is equal to 16. Great job guys. It is now time for your independent task. Where you're going to be looking at some more addition and subtraction problems. I want you to complete the equations that you've been given by using your known facts You're known number bonds within 10 to derive new facts. While you're doing your sheet, can you spot any patterns in the numbers? Please pause the video now to complete your task and resuming the video once you are finished. Great, let's go through these answers. 46 take away 40 is six. 46 take away 30 is 16. 46 take away 20 is 26 46 take away 10 is 36 46 add 10 is 56 46 add 20 is 66. 46 at 30 is 76. 42 take away 40 is two. 42 take away 30 is 12. 42 take away 20 is 22. 42 take away 10 is 32. 42 add 10 is 52. 42 add 20 is 62. 42 add 30 is 72. So in all of these equations, you'll notice that the number that I'm taking away or that I'm adding to the part that I'm subtracting or adding. The tens, the number of tens I have is changing. Therefore, the number of tens in my answer is changing. So because we're subtracting and adding tens that is changing the number of tens that are in my answer. Let's look at these next ones. 26 is equal to 66 take away 40. 36 is equal to 66 take away 30. 46 is equal to 66 take away 20. 56 is equal to 66 take away 10. 76 is equal to 66 add 10. 86 is equal to 66 add 20. 96 is equal to 66 add 30. 25 is equal to 65 take away 40. 35 is equal to 65 take away 30. 45 is equal to 65 take away 20. 55 is equal to 65 take away 10. 75 is equal to 65 add 10. And 85 is equal to 65 add 20. And 95 is equal to 65 add 30. The multiples of 10 that I'm adding or subtracting have different numbers of tens. And therefore my answer has different numbers of tens. So that is affecting the number of tens that are in my answer. Team, you've done a fantastic job today with all of your learning. I was so impressed with the way that you used mental strategies to add and subtract tens from two digit numbers. Well done and see you soon in another session. Bye bye. If you'd like to please ask your parents or carer to share your work on Instagram, Facebook, or Twitter, tagging @OakNational and #LearnwithOak. All there's left for you to do now guys, is complete your quiz. So when the video has ended, make sure you take part in that to recap everything you've learned in today's session and to show off everything that you have remembered. Good luck with it.

Adding and subtracting tens from a 2-digit number

Adding and subtracting tens from a 2-digit number

Slide deck

Lesson details

Key learning points

Licence

Video

Worksheet

Starter quiz

4 Questions

Exit quiz

5 Questions

Lesson appears in

UnitMaths / Addition and subtraction of 2-digit numbers

Maths