Adding two 2-digit numbers

Key Stage 1

Maths

Addition and subtraction of 2-digit numbers

If I know that 6 - 2 - 3 = 1 then 60 - 20 - 30 = ____

Seventy Two subtract Thirty is equal to ______

﻿Hello everybody, and welcome to today's math session. My name is Ms. Hughes, and today's objective is going to be adding two, two-digit numbers together. So let's get started. Let's have a look at our lesson agenda for today then. We're going to start by adding two-digit numbers, and using some known facts to help us with this. Then we're going to go for a talk task, then we're going to find some new number facts that are going to help us add two, two-digit numbers. And finally, there's a main task. And of course, a quiz at the very end for you to have a go at. For today's lesson, you are going to need a pencil and a rubber, and some paper. So pause the video now to go and get these things, if you have not. Fantastic, let's move on. So to work our brains for this session, we are going to solve these equations using known facts. So I want you to look at each of the equations where we have a two-digit number, and you're adding a multiple of 10. And I want you to think about your known facts within 10 that you can use to help solve these equations. Off you go. Great, let's look through these answers. So, these are the first set. If I've got 26 add 30, I know I've got two tens and three tens. So the fact two add three is equal to five is going to help me solve this one. Three add six equals nine is going to help me solve this one with 32 add 60. Five tens add four tens, so 55 add. Five add four equals nine will help me solve this one because I've got five tens and four tens in the numbers 55 and 40. And finally, four take away two is equal to two, is the number fact that will help me solve this equation because I've got four tens in 44 and two tens in 20, and I need to take the two tens away. So these are the answers that you should have gotten, 56, 92, 95, and 24. Let's have a look through the next lot. So two add three equals five is the known fact that would help you with 28 add 30. Three add six equals nine is the known fact that was going to help you add 39 and 60 together because you've got three tens and six tens. With 77 take away 40, seven take away four equals three is the known fact that would help you with that. And 64 take away 20 is equal to 44 because six take away two is equal to four. So we were able to work out what six tens take away two tens equals. So these are the answers, 58, 99, 37, and whoops, there's a little mistake there, shouldn't be 84. I've thought we've added them, but it's taken away. So this one should be 44. It's always really important to check your symbols when you're doing your answers. Okay, let's move on, guys. So, we're going to start off our lesson by thinking about some known facts that are going to help us solve some number problems like this. So let's read the problem. On Monday, 63 people were in their homes, and 24 people were on the bus. How many people were there altogether? To help us solve this problem, let's look at a part-whole model like this one. Okay, when we're reading or when we're solving a word problem like this, it's really important that we think about the information that we know. So what values do I know already? Do I know any of the values of my parts? Do I know the value of my whole? And also I need to think about what am I trying to figure out, what part is missing from my part-whole model? Well, from this question, I can tell that I know. I can tell that one of my parts is 63 because there were 63 people in their homes. So let's put 63 in my first part. Another part is 24, because there are 24 people on the bus. So that goes in my second part. So I know those two values. What I don't know is the value of my whole. I don't know how many people there are altogether. To solve that though, I need to add these two together. So let's have a look at what that looks like. Okay, so I've added my two parts together and put them in my whole. So let's count them. 10, 20, 30, 40, 50, 60, 70, 80, 81, 82, 83, 84, 85, 86, 87. So I started with counting my tens first and then my ones, and altogether, there are 87 people. So the equation we just did was 63, which was one of my parts, add 24, which was my second part, equals our whole, 87. We could have solved this in another way using some number facts. So remember, we had our two parts, 63 add 24, and we were trying to work out what our whole was. So we could have started by partitioning our two parts. 63 is partitioned into six tens, which is 60 and three ones, and 24 is partitioned into two tens which is 20 and four ones. Now that I've partitioned in my numbers, I can think about my known number facts that are going to help me add them together. So let's start with our tens. I have six tens and two tens. So if I know that six add two is equal to eight, then I also know that 60 add 20 is equal to 80. Now that I've added my tens, I can think about my ones. And I know that three add four is equal to seven. So I've got 80, I've got eight tens and seven ones when I've added my two numbers together. So now all I need to do is add those bits together, and that will give me my answer. Let's look at another one. On Tuesday, 52 people were in their homes and 35 people were on the bus. How many are there altogether? We're going to get to our trusted part-whole model again for this one. And we need to think, what parts do we know already? What values do we know? And what are we trying to figure out? We know that we're trying to find out how many people there are altogether. So that tells me we're looking for a whole. So I don't know yet how many people there are altogether, I don't know my whole. What I do know is the value of my first part which is 52, and the value of my second part which is 35. Remember, I need to add to my two parts together to get my whole, so let's see what that looks like. There we are, let's count up how many 52 and 35 together equals. 10, 20, 30, 40, 50, 60, 70, 80, 81, 82, 83, 84, 85, 86, 87. So my whole is 87. There are 87 people altogether. And I can represent what we've just done in an equation. 52, which was my first part, add 35 which was my second part, gives me 87, which is our whole. Remember, we can also use some known facts to help us with this. So I'm going to show you again, how we would do that. We could have partitioned our two parts, 52 and 35 into tens and ones. So 52 is partitioned into five tens, which is 50 and two ones, which is two. 35 is partitioned into three tens, which is 30, and five ones, which is five. Now that I've partitioned our parts, I can think about the no number facts that we have that are going to help us solve this problem. Let's start with our tens. I know that I've got five tens and three tens. So if I know that five add three is equal to eight, then I also know that 50, sorry, add 30 is equal to 80. So I know that my tens added together equals 80. Now I can look at my ones. I know that two add five is equal to seven. So I know that my ones added together is equal to seven. Now I just need to add my tens and my ones together to get my final answer. Right team, it's time for you to have a go at this with your talk task. So you can see that you've got a number of different equations on the board, there are six of them where you're adding two, two-digit numbers together. And I want you to think about the known facts that you have that can help you to add them together. So I want you to think about the known facts that will help you at the tens, and the known facts that you have that will help you add the ones. Let's go through an example together and then you can do the rest of them yourself. We'll go through this first one, 31 add 42. So, I'm going to start by writing out my equation, 31 is my first part, add 42, which is my second part. And I don't know what my whole is, so I'm just going to put my equal sign and leave that part blank. Okay, let's start by looking at our ones. So I know that 31 has one, one and 42 has two ones. So the number bond for the ones is one, ones add two ones is equal to three ones. And you can see that I'm using the sentence structures at the bottom to help explain what I'm doing. Now that we've looked at our ones, we can look at our tens. So underline those now. So I've got three tens in 31 and four tens in 42. So the number bond for the tens is three add four is equal to seven. If I know that three add four is equal to seven, then I know that 30 add 40 is equal to 70, like this. Brilliant. You are going to have a go at the rest of this talk task in our team. So pause the video here to have a go at your task, and play the video when you are ready to continue. Okay team, in this half lesson, we're going to think about some more addition problems, and think about some new facts that are going to help us to add these two-digit numbers together. So let's start with this one on the board. On Monday, 63 people were in their homes and 24 people were on the bus. How many people are there altogether? I'm going to start by thinking about my part-whole model, okay? So what values do I have already? And what values are missing from this word problem? Well, we're trying to find out how many people there are altogether. So I know that we don't yet have our whole because I don't know how many people there are altogether. I do have one of my parts though, which is 63, so I'm going to put that in here. And I have the parts 24, another one of my parts in here. I know that if I want to find out this missing whole, I need to add my two parts together. So let's think about our known number facts that are going to help us to solve this problem. Let's think about our ones first. So we know that we've got three ones in this part 63, and four ones in this part 24. I know that three add four is equal to seven. So we know that there are going to be seven ones altogether. So let's move our seven ones into our whole. Now we've got our tens left. I can see that I've got six tens in this part, and two tens in this part. If I know that six add two is equal to eight, then six tens add two tens is equal to eight tens. So 60 add 20 is equal to 80. So we know that we have 80 altogether is the value of our tens. There we go. Now we can add the value of our tens, 80, and the value of our ones, seven, together. And that will give us 87, because I know that 80 add seven is equal to 87. Let's have a look at one more. On Tuesday, 52 people were in their homes and 35 people were on the bus. How many people are there altogether? So I can see that one of my parts is 52. So let's put 52 in one of our parts. And another part, our second part is 35. I don't yet know what my whole is, because I don't know how many people there are altogether, but I do know that I need to add my two parts together to make my whole. So let's think about the number facts that we can use to help us do that. Let's think about our ones first. So I have two ones in this part, and five ones in this part. And I know that two add five is equal to seven. So I know that we have seven ones altogether. Let's put them in the whole there, there's our seven ones. Now let's look our tens. I can see that I've got five tens in this part left, and three tens in this part left. If I know that five add three is equal to eight, then five tens add three tens is equal to eight tens. So that means 50 add 30 is equal to 80. We can move our 80 tens into our whole. Now we have our eight, so we can move our eight tens which equals 80 into our whole. Now that we have our 80, so our eight tens have the value of 80 and our seven ones, we can add those together to get our whole. So I know that 80 add seven is equal to 87. So our whole is 87. Okay, guys, it is now your time to do the independent task. So in your independent task, you're going to be adding two, two-digit numbers just like we were doing then in our word problem. So I want you to complete the equations that are on your sheet by using the known number facts that we were just using to derive new facts. I would also like you to complete a part-whole model pictorially so you can draw your tens and ones, just as we did in our earlier examples. When you're going through this sheet and you're going through your answers, I want you to see if you can spot any patterns in the numbers as well. Before you get started, let's just do one example. So, my equation is 41 add 25. So I've got two parts, 41 and 25, and I've drawn them in my parts here. So I've got 41 is my first part, and 25 is my second part. I know that I need to add my two parts together to get my whole, so I've put my two parts in my whole here as well. So I've drawn out my equation pictorially. Now it's time to think about my number facts. If I think about my ones first, and I thought I've got one, one and five ones here, and I know that one add five equals six. So I know that altogether, I've got six ones in my whole. Now I can look at my tens. I've got four tens in this part, and two tens in this part, you can see them here. Four tens and two tens. If I know that four add two is equal to six, then I also know that 40 add 20 is equal to 60. Therefore I'm left with 60 because I've got six tens and six ones. And I know that 60 add six is equal to 66. So my answer is 66. This is the sort of thing I would like you to be doing for your independent task where you're drawing your equation out pictorially on a part-whole model, and you are using your number facts to help derive new number facts. Pause the video now to complete your task, and resume the video once you are finished and ready to continue. Brilliant, let's have a look at those answers. Well done on all of your fantastic hard work today. I thought you did a great job with adding two, two-digit numbers and finding the mental strategies to help you with that. So well done. See you very soon. Bye-bye. If you would like to, please ask your parent or carer to share your work on Instagram, Facebook, or Twitter, tagging @OakNational and #LearnwithOak. It is now time to complete your quiz. So when the video's ended, don't forget to give it a go to see everything you've remembered from today's session.

Adding two 2-digit numbers

Adding two 2-digit numbers

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UnitMaths / Addition and subtraction of 2-digit numbers

Maths