Addition and subtraction within 20
Current time:0:00Total duration:4:03
Voiceover:Let's see if we can compute what 14 - 2 is, and then figure out what 14 - 4 is, and then figure out what 14 - 6 is. And I encourage you to pause this video and try to figure these out before I work through them. Voiceover:So I'm assuming you've given it a try, now let's think about it. The number 14 just by how it's written, we know that it's going to be 1 group of ten. That's what this 1 tells us plus another 4 ones. So let's verify that we have 14 objects down here. This is 1, 2, 3, 4, 5, 6, 7, 8, 9, 10. So we definitely have a group of ten. We have 1 group of ten here, so that's my group of ten. So let me box that off, so I have my group of ten. And then how many ones do I have? I have 1, 2, 3, 4, I have four ones. So this is indeed, 14. This one that we have right over here, this represents this ten, it's 1 group of ten. This is 1 group of ten, let me write that down. Group of ten. And then we have 4 ones, 4, I guess we could say, 4 ones like that, so that is indeed 14. Now let's look at each of these, what is 14-2 going to be? Well we could take away 2, so take away 1... take away 2, and how many are we left with? Well we still have our one group of ten, so it's going to be 1, followed by how many ones do we have left over, well we have 2 ones. We have 2 ones left over. 1, 2, so we are going to be left with 12. Now what about 14 - 4? So let me clear this out. Actually let me do it like this. So if I clear that, so what is 14 - 4 going to be? So now I'm going take away 1, 2, 3, and 4. I have essentially taken away all 4 ones. So what am I left with? Well I still have my 1 group of ten, I still have my 1 group of ten and I have 0 ones left over. No ones anymore, so now I have 0 ones. And so 14 - 4 is 10, and that makes sense. 14 is 10 + 4 and now we're subtracting 4, to get 10, let me write this down. This is the same thing, 14 is 10 + 4. Voiceover:10 + 4, this is 14 and then we're going to subtract 4, Then we're subtracting 4, so if you have 10 + 4 - 4. Well the 4 - 4 is going to be 0, you're going to be left with 10 + 0 or just 10. We can do the same thing up here. This is equal to 14 is 10 + 4 and then we subtracted 2. And then we subtracted 2. So what you're going to be left with 10 + what's 4 - 2? It's 2, so that's what we got right over here. Let me make this very clear, this simplifies to 2. This 2 right over here. This simplifies to 0, this 0 right over here. Now let's do the last one, what is 14 - 6? Well we're going to take away 1, 2, 3, 4, then 5, and 6, so now we've broken into our group of ten. So this is going to be a one-digit number. And we are left with 1, 2, 3, 4, 5, 6, 7, 8. So this is going to be equal to 8 and we're all done.