Consolidating addition and subtraction of 2-digit numbers

Key Stage 1

Maths

Addition and subtraction of 2-digit numbers

Seven add two add three is equal to _____

﻿Hello everybody and welcome to today's math session. In this lesson, we're going to be consolidating, adding and subtracting two digit numbers, so let's get started. Let's take a look at today's lesson agenda, we're going to start off by revisiting adding two, two digit numbers together. Then we're going to be looking again at subtracting two, two digit numbers. Next we'll be thinking in more detail about using some mental strategies that will help us to do this. And finally, you'll have your final quiz. For today's lesson you are going to need a pencil and the rubber could be useful and some paper. So pause the video now to get those things if you have not got them already. Brilliant, let's get started. To begin our lesson we're going to use a bit of a... or have a bit of a brain warm up by solving these equations, using your known facts. So two equations are 42, add 36 and 59 take away 37. And I want you to think about the known fact that you already have within 10. They're going to help you derive these equations, off you go. Brilliant, let's take a look at them. So, the first thing I'm going to do is think about the ones that we have in these two equations. So here I've got two, ones and I'm adding six, ones and here I have nine, ones and I'm taking away seven, ones. I know that two add six is equal to eight and I also know that nine subtract seven is equal to two. So now in this answer, I'm going to have eight, ones and in this answer, I'm going to have two, ones. Now let's have a look at the tens. I know that four add three is equal to seven and that's useful here because I've got four, tens, add three, tens. I also know that five take away three is equal to two and that's useful because I've got five, tens takeaway three, tens. This is going to help me derive my number facts from multiples of tens I have. So, if I know that four add three is equal to seven and five, takeaway three is equal to two. And I also know that 40 add 30 is equal to 70 and 50 takeaway 30 is equal to 20. Now I can figure out my answer and this one is going to be 78 and this one is 22. So 42 add 36 is equal to 78 and 59 takeaway 37 is equal to 22. Great job guys if you found those known facts and you've got those answers well done. Let's move on, we're starting off today's lesson with revisiting adding two, two digit numbers. So we're going to read this question where we're going to need to add our two digit numbers. On Wednesday, 24 people were in their homes and 75 people were on the bus. How many people are there all together? Let's have a look at this question in a part-whole model, okay. So I need to think about what parts I already know, what values I already know, about our whole model and I also need to think carefully about what I'm trying to find out. So I know that one of my parts is 24 because there were 24 people in their home. I know that one of my parts is 75, because 75 people are on a bus and I'm trying to figure out how many people there are all together. But what I don't know yet, is the value of my whole, that is still unknown, we don't know yet. How many are there altogether. But I'm going to fill out my part, so 75 is one part 25 is another part, I'm going to put a question mark in my whole because we don't yet know what that is. As an equation to work this out, it would be like this 75, which is one part add 24, which is another part is equal to, then I've got my question mark for my whole, we don't yet know what that is. Okay, let's have a think about this equation in more detail then, and think about the strategies we can use to solve it. So here we have a representation of our equation, I've got my two parts, 75 and 24, and I still don't yet know my whole, so that's empty. I know that if I want to find out my whole, I just have to add them both together. So let's have a think about the known number facts that we can use that are going to help us to solve this problem. We can start by partitioning our parts, so I've got 75, which is partitioned to seven, tens or 70 and five, ones. 24 can be partitioned into two, tens, which is 20 and four, ones, which is four. I can think about our known facts that can help us add our ones together now. I know that five add four is equal to nine, if I was to add these five, ones and these four, ones together, I know we're going to end up with nine, ones in our answer. Let's have a look at what that looks like in our part whole model. I've got my five, ones and my four ones and I'm left with nine, ones in my whole like this. Now we can look at the tens. I know that seven add two is equal to nine. That helps me to know that seven, tens add two, tens would give me nine, tens. Therefore 70 add 20 will give us 90. Let's see that looks like on our part-whole model here, so I have my 70 add 20 leaves me with 90 or nine, tens altogether. Let's count now how many we have altogether in a whole, 10, 20, 30, 40, 50, 60, 70, 80, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99. I also know that my answer's going to be 99 because I've got 90 here with our tens, I've got nine, tens. And I have nine, ones and I know that 99 is equal to 99. The final whole is 99. Okay guys, it's now time for your task, you will see that you've got four different addition equations, where you are going to be adding two, two digit numbers together. I want you to work out what the answers are for each of these, by thinking about your know number bonds or no number facts that will help you do this mentally. I would like you to use these sentences as I did in our last example, to help explain the process. So pause the video now to have a go at this task and resume the video when you're ready to continue. Okay, let's go through these then. So let's look at this first one 71 add 32 is equal to, well, the number bond for the ones is one add two is equal to three. The number bond for the tens is seven add three is equal to 10. Therefore I know that 70 ad 30 will be, so 100, so my answer is going to be 103. My next one for the ones my number bond is three, add five is equal to eight. My number one for the tens is four add two is equal to six. Therefore 40 add 20 is equal to 60, my final answer will be 68. This one, my number ones for the ones is eight add one, which is equal to nine. My number one for the tens is to add five, which is equal to seven. I know therefore the 20 add 50 is equal to 70 and my fine answer will be 79. Let's have a look at this final one then. My number bond for the ones is four add two is equal to six, my number bond for the tens is six add three is equal to nine and therefore I know that 60 add 30 will be equal to 90. So my final answer is going to be 96 here. Great job guys, really hard work so far. Let's move on now to our subtraction of two, two digit numbers. So again, we're going to look at a word problem, just like we did with our addition and think about the known facts we can use that will help us to work this out quickly in our heads. On Thursday, 66 people were in town and 32 people left for work, how many people were still in town? Let's look at this on a part-whole model. Remember when we have a word problem like this, we need to think carefully about what values of our parts do we know, or what part do we know already and what parts are we still trying to figure out? Well, because firstly I start with 66 people in town and then 32 people leave work and I'm working out how many people are left, I know that my whole is 66, I also know that my first part is going to be 32, 66 is my whole, 32 is my first part. I still don't know what this part is yet. I need to figure out what this missing part is. As an equation, we will be doing this to work out our missing part. 66 our whole, take away 32, our first part, will give us our unknown second part. Let's look at this in more detail then. Okay. So I started off with 66 in the whole because remember we've got 66 as a whole and we're taking away our first part and let's see what it looks like on this part-whole model If I was to take away 32 from this whole let's take them away, probably go to 32 is in one of my parts and what's leftover, is this amount let's count that value, 10, 20, 30, 31, 32, 33, 34. I can see that my answer is going to be 34, but let's have a look at the strategies or the number of facts we could have used to help us work this out. Remember we could partition our whole 66 into six, tens, which is 60 and six, ones, which is six and we can partition our first apart 32 into three, tens which just 30 and two, ones, which is two. Now I can look at the number of facts for my ones and for my tens and then add those final answers together to get my final answer. My ones, I know that six take away two is four, therefore I'm going to have four, ones in my answer. Taking away the tens now I know that six, tens take away three, tens is going to be three, therefore, 60 take away 30 is going to leave me with 30. Now I'm left with 30 as a value of my tens and four, ones let's add them together, 30 add four and we left with 34. And as you can see that matches up with our numbers here in our part whole-model adds up with our tens and ones. Okay, I'm going to get you now to have a go at the similar task that you did for the audition, but with subtraction now. So you've been given four subtraction sums and I want you to think about the number bonds that you can use for the ones and for the tens that will help you work out this answer mentally. So pause the video here to have a go at this task and play the video when you're ready to receive. Okay team, let's go through these answers then. So, I know that my number one for the ones is two take away one, which is one. My number one for the tens is seven take away three, which is four. So 70 takeaway 30 is 14, so I know my ones would be 41. The answer for this one, I know that five take away four is one, four takeaway two is two. So 40 take away 20 will be 20, my answer's going to be 21. I'm now just going to go through the answers of these ones, So my answer here is going to be 37 and my answer here is going to be 32, well done if you got those and also well done if you use these sentence. Okay team, we're going to now think a little bit more about exploring different mental strategies that we can use when we're adding or subtracting two, two digit numbers from one another. I want you to have a look at the blue box at the bottom of the screen, which says, if I know then I know. And I want you to think carefully about how I'm using those sentences or those sentence starters rather, to answer these equations in my head, thinking about those mental strategies and number bonds that we can use to help us. Okay, so let's start with the first one, 37 add 52. We are looking for our whole by adding out two parts together. I'm going to think about the tens first, if I know that three add five is equal to eight. Then I know that three, tens add five, tens is equal to eight, tens. Therefore 30, that should be an add, add 50 is equal to 80. So I know that I'm going to have 80 or eight, tens in my whole. Let's look at the ones now. If I know seven add two is equal to nine, then I know that nine, ones is going to be in my answer as well. So I'm left with eight, tens, which is 80 and nine, ones I need to add those together now so 80 add nine, equals 89, so my answer to this equation is 89. Let's have a look at this next one, 47 take away 25. If I know that four takeaway two is equal to two, then I knew that four, tens that we've got here and two, tens that we've got here, if I subtract them, I will have two, tens in my answer. Therefore, 40 take away 20 will leave me with 20. Now I can think about my ones. Seven take away five is equal to two. So I'm going to have two ones in my answer. So now I'm left with two, tens, which is 20 and two, ones. I can add them together, which gives me 22. So I know that my answer for this equation, 47 take away 25 is going to be 22. Rightly, it's now time for that main task of yours, so you are going to take onboard what we've done in today's lesson and thinking carefully about those number bonds that are going to help you solve equations. And I would like you to solve these three word problems. Remember when you are answering word problems, you need to think very carefully about what parts you have already and what values you already know. And you need to think about what you're trying to work out, what part or value is missing. Pause the video to complete your task and resume the video once you've finished. Great effort team, I'm really impressed with how hard you worked for that main task. Let's have a look at the answers now, Right, on Friday 67 people were in town and 24 people left for work, how many people were still in town? This is going to be a subtraction equation, so I'm doing 67, which is my whole number of people that I started with, takeaway 24, which is my first part and that will leave me with 43. Next, 56 people were in town and 12 people were in their homes, how many people were there altogether? I can tell that we're going to be adding these two together the number of people in town and the number of people in their homes, nothing is being taken away. So this should be... So this should therefore be an addition sum. So 56 add 12 is equal to 68, my answer's going to be 68 for that. On Wednesday 56 people were in town and 12 people left for work, how many people were still in town? This is a subtraction equation because I start with 56 people, then 12 leave, they get taken away. My equation for this question should be 56 take away 12 is equal to 44. So for this bottom question, the answer is 44, this middle one, the answer is 68 and this top one as I said already is 43. Well done for getting those and great job if you used or if you derived your facts with your known facts that you already had. Team, thank you for all of the hard work that you put into today's session. I was really impressed with the way you remembered the mental strategies that you could use to add and subtract two digit numbers. Good luck with your quiz and hopefully see you on a session soon, bye bye. If you'd like to, please ask your parent or carer to share your work on Instagram, Facebook, or Twitter tagging @OakNational and #LearnwithOak. Now guys, it's time for you to complete your quiz. So good luck.

Consolidating addition and subtraction of 2-digit numbers

Consolidating addition and subtraction of 2-digit numbers

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