Sal subtracts 14 - 6 by first thinking about subtracting 2 and 4. Created by Sal Khan.
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- I am struggling to find a video on Subtracting within 20 with regrouping. I dont want to use blocks, I don't want to use visualizations. I want to know how to know how to write subtract 9 from 16 with regrouping.(7 votes)
- Try this,
We have the original number, but we can put it in expanded form*.
So, 16 = 10 + 6, and we can use this.
We know that 10 - 9 = 1, and then we can add the remaining 6 to get 7.
Expanded form is just taking the values of each number and adding them to create a number. For example, 17 = 10 + 7
You can also use this for bigger numbers, like
1,234 = 1,000 + 200 + 30 + 4(18 votes)
- Hi there,
Could you please tell which video explains how to do 9-=5 and -4=5.
Any help on this is much appreciated.(5 votes)
In these type of problems all you have to do is do the opposite of what it's asking. But, you would, in this case, still subtract since you are trying to find the number you subtract by and not the number you subtract from. So, 9-5=4 the missing value is 4.
For the second problem you add since you are trying to find the number you subtract from. So, 4+5=9 the missing value is 9.
Hope this helps!(1 vote)
- What is the commutative property? Is addition commutative?(1 vote)
- It is when you can change the order of an equation and get the same answer. For Ex: 4+3= 7 is the same as 3+4=7(0 votes)
- If your subtracting by adding a negative number does that lead to borrowing to significant to basic subtraction?(0 votes)
- The basic concept of adding a negative number is exactly the same as subtracting that number. In higher math this may not always be the case but you will learn about those types of situations later. For now think about adding a negative number as the same process as subtraction. For example 10 + -3 = 7 and 10 - 3 = 7.(0 votes)
- so people can think of the four's cancelling each other out in the second problem and being left with only ten?(0 votes)
- Yes, but not all math problems are simple like that. But it is definitely not encouraged to think as such, as people might think that 14 - 4 = 1 if they cancel out the 4 and forget that there is a 0 behind the 1.(0 votes)
- Why is subtraction not commutative?(0 votes)
- An operation is commutative if you can change the order and get the same answer.
2 - 1 = 1
1 - 2 = -1
Therefore, subtraction is not commutative.(0 votes)
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.