- Worked example: Subtracting 3-digit numbers (regrouping)
- Subtracting 3-digit numbers (regrouping)
- Worked example: Subtracting 3-digit numbers (regrouping twice)
- Worked example: Subtracting 3-digit numbers (regrouping from 0)
- Subtract within 1000
Sal uses regrouping (borrowing) and place value to subtract 629-172. Created by Sal Khan.
Want to join the conversation?
- If it was 29-72,how would you borrow?(3 votes)
- Actually, this get's more complex in numbers. There are such numbers that are NEGATIVE. These are numbers below 0. If it is -12 degrees in winter, it is VERY cold! You can watch Sal's videos on negative numbers to learn more.(2 votes)
- I so confused! can you subtract negitave numbers!(8 votes)
- You can subtract negative numbers. But, to subtract negative numbers, you have to learn a few special rules.
When subtracting a negative number, the negative number being subtracted becomes positive.
4 - (-)3 (Because we are subtracting a negative number -3, the number becomes +3.)
4 + 3 = 7
When subtracting a positive number from a negative number, you do something similar to adding.
-4 - 3 = -7 (Instead of ending up with -1, I ended up with -7. However, -7 is actually worth less than -1.)(7 votes)
- can you make these easier?(4 votes)
- at1:27, do you have to take away 100? or can you take away just 1?(3 votes)
- Well,it depends on what strategy you are using to subtract.If you're just doing standard algorithm then, you would just write that you are borrowing "1" but if you want to say the accurate value of the number then you would say that you're borrowing 100, which is what you are actually borrowing!
Hope this helps!(0 votes)
- Why do you need a second video for regrouping(2 votes)
- This is just practice to help. You don't necessary "need" any video if you're confident with her math.
But this video can also strengthen your regrouping skills and your subtracting 3-digit numbers.
Later, this will help you in math, eventually. =)(2 votes)
- When Sal writes the second line in the second problem, why does he put a parenthesis around the bottom row?(1 vote)
- Sal wrote the second number in expanded form. But we are subtracting the second number, so we need to put the subtraction sign. He could have put negatives for each place value, but that loses the sense that we have expanded the number.(3 votes)
I've written the same subtraction problem twice. Here we see we're subtracting 172 from 629. And all I did here is I expanded out the numbers. I wrote 629 as 600 plus 20 plus 9, and I rewrote 172, the one is 100. So that's there. This is 7/10. It's in the tens place, so it's 70. And then the 2 is 2 ones, so it just represents 2. And we'll see why this is useful in a second. So let's just start subtracting, and we'll start with the ones place. So we have 9 minus 2. Well, that's clearly just 7. And over here we could also say, well, 9 minus 2, we have the subtraction out front. That is going to be 7. Pretty straightforward. But then something interesting happens when we get to the tens place. We're going to try to subtract 2 minus 7, or we're going to try to subtract 7 from 2. And we haven't learned yet how to do things like negative numbers, which we'll learn in the future, so we have a problem. How do you subtract a larger number from a smaller number? Well, luckily we have something in our toolkit called regrouping, sometimes called borrowing. And that's why this is valuable. When we're trying to subtract a 7 from a 2, we're really trying to subtract this 70 from this 20. Well, we can't subtract the 70 from the 20, but we have other value in the number. We have value in the hundreds place. So why don't we take 100 from the 600, so that becomes 500, and give that 100 to the tens place? If we give that 100 to the tens place, what is 100 plus 20? Well, it's going to be 120. So all I did, I didn't change the value of 629. I took 100 from the hundreds place and I gave it to the tens place. Notice 500 plus 120 plus 9 is still 629. We haven't changed the value. So how would we do that right over here? Well, if we take 100 from the hundreds place, this 600 becomes a 5, 500, and we give that hundred to the tens place, it's going to be 10 hundreds. So this will now become a 12. This will now become a 12. But notice, this 12 in the tens place represents 12 tens, or 120. So this is just another way of representing what we've done here. There's no magic here. This is often called borrowing, where you say hey, I took a 1 from the 6, and I gave it to the 2. But wait, why did this 2 become a 12? Why was I able to add 10? Well, you've added 10 tens, or 100. You took 100 from here, so 600 became 500, and then 20 became 120. But now we're ready to subtract. 12 tens minus 7 tens is 5 tens. Or you could say 120 minus 70 is 50. And then finally, you have the hundreds place. 5 minus 1 is equal to 4, but that's really 500 minus 100 is equal to 400. 500 minus 100 is equal to 400. And so you get 457, which is the same thing as 400 plus 50 plus 7.