Learn about Basic Subtraction

Khan Academy Presents: Learn about Basic Subtraction

Learn about Basic Subtraction Welcome to the video on basic subtraction. So what is subtraction? Subtraction is essentially taking away from something else. So you learn addition, so let me see in addition and I think used avocado as the example but let’s use something else. I’m using colors. Let’s do bananas where we use yellow. So, we learned in addition in basic addition that if we have one, two bananas and then we were to grow three more bananas and I’ll make the three new bananas green because when you grow banana has a chance to ripen its green. So let’s say I would to grow three more bananas and they look more like beans than bananas but I think you get the idea. So we learn in basic addition that the total number of bananas we would now have would be two plus the three new ones that we just grew plus three. And you realize that’s five. And the other way to visualize that was if we look at a number line— let me draw at thicker number line in that. Let’s say that I’ll do it in magenta, this colors is magenta. So is you’re looking number line, and this is zero bananas, this is one, two, three, four, five, six and it just keeps going into a larger and larger numbers. So, we said we start at off at two bananas and we grew three bananas, so that took us three steps larger on the number line or three steps to the positive on the number line so that would be one, two, three so we got this far. So what’s the total number of bananas we have we have five, we start at two and we added three. Okay, so that as a review of addition, so what is subtraction? Well, subtraction is the opposite of addition when you’re adding two numbers at least we’ll deal with the positive numbers right now. I don’t want to scare you too much at you tender age but actually I don’t how old you are because this is a video. But if we’re dealing with positive numbers when you add two positive numbers, you get a larger number. When you subtract two numbers you’re going to get at least for now you’re going to get smaller number, so lets’ think about subtraction. So, let’s start— maybe I’ll go back to avocado because there easier to draw than bananas. So let’s say I were to start with, now let’s say six avocados, how many is that, one, two, three, four, five, six avocados. So, I start with six avocados, start six avocados, I start with six avocados and let’s say I’m very hungry ad despite the fact that avocados are calorie rich food I decide top eat four avocados, so I eat four avocados. My question to you is how many avocados do I have left, let’s see these are my six avocados, one, two, three, four, five, six, let’s say that this one looks particularly tasty and ripe and ready to eat so I eat that one, so let’s take that one away. That doesn’t excess anymore so we’re left with five. I told you I eat a total of four avocados and let’s say that this one also looks awfully good so I eat this one— I’m just going right to left. I could just say this one looks good. It doesn’t matter which avocado I pick. They are all equally delicious. Let’s say I’ll pick— well, I’m going to start go from the right and go left just so you have some visualization of things. Then I pick this avocado and I eat it and I said four avocados how many I eaten so far? I eaten one, two, three avocados so I’ll have to eat more— you already get my daily allowance of avocados. So I eat four avocados. So the question was how many avocados do I have left? Well, look here I had six I eat four of them ad right here I have two uneaten avocados. So left, the avocado’s left is going to be two avocados -- I should probably; you’re just probably learning to read right now so I probably use words easier to spell than avocados but never hurt to be exposed. So we just did subtraction, if I were to writer this without having to draw avocado we would’ve said six minus, you could view it as minus taking away. So six minus four avocados is equal to two avocados, if we go to the number line, let’s think about that number line. Hopefully you’re reasonably familiar with this point. Let’s look at that number line, so let’s that this is zero, one, two, three, four— I don’t want to bore you, five, six and you just keeps going, I could extend this number line because there is no largest number as you will learn later or as you can imagine. If there is a largest number I could always think of a bigger number because I could add one to that number. But anyway let’s go back to the problem and not get too deep with how large number can get. So how many avocados that we started off with, we started off with six avocados. And we ate four or we could say we subtract at four, now remember when we added, if we added four avocados let’s say if we grow more four avocados we would’ve go to the right, four space of the number line. But with subtraction you go to the left on the number which makes sense you’re taking away. If I ate one avocado I would have five left, if I ate two avocados I would have four left. So the number of avocados is that eat is the number that I take a jump to the left. So, since I ate four avocados so I ate one, two, three, four avocados, so I will go spots to the left when I ate my four avocados and I will be left with two avocados. Let’s do a couple of more examples and I think you’ll have a descent idea of how subtraction works. Well actually, —and let’s think of it this way, let’s say I want you to show how, I want to see how addition and subtraction are related to each other. Let’s say that I was to tell you that I started with six avocados. Let me to think about, the best way words to say— let’s say that I started with six avocados. Let me just erase this. I don’t want to get too crowded on the page— let’s say I start with six avocados just like the problem and let’s say someone comes and eats a bunch and I’m left with two avocados. I’ll do it in different colors. Let’s say I’m left with two avocados. My question is how many did the person eat? How many did they eat? I don’t know, how many did they eat? I started with six and I’m left with two, how many did they eat? Well there’s a couple of ways to think about, you could say six avocados minus the amount eaten is equal to two, so the way you have to think about this is six minus what is equal to two, so let’s think about it from the number line or at least we can even draw the avocados— I still draw in green— one, two, three, four, five, six, so how many do I have to eat, how many do I have to take away to be left with two. Let’s say these are the two that I’m left with, so I would have to take away these four. So, essentially that person would’ve have eaten four avocados. Another way you could think about it is what is the difference between the number of avocado I started with and the numbers of avocados that I’m left with? So the way you think about differences how different is six and two and that is how many must have been eaten and so you could actually write like this. The difference between six and two is equal to the number of avocados that are eaten and let me draw you a little graph that might make a little more sense of that. Let’s say that that bar represent six and you know it could be six of anything but in our the example is six avocados. And lets this bar here represents how many were left, let’s ay that’s two. So, our question is how many were eaten? So this amount right here had to be what was eaten and so could view it has the difference between six and two or four avocados. Anyway, I hope I didn’t confuse and I hope that was reasonable introduction to subtraction. Let me know if you found this daunting but I will see you in the next video.