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### Course: Early math review>Unit 3

Lesson 4: Hundreds

# Regrouping whole numbers: 675

Sal regroups 675 into various addition problems. Created by Sal Khan.

## Want to join the conversation?

• I'm not understanding the concept of regrouping whole numbers in this video lecture. Are subtracting from these whole numbers to simplify the equation? I'm lost on this information. Crossing the numbers out and throwing lower numbers don't tell me anything. I throw my hands up in the air.
• So I think where people get confused on this is that they're trying to figure out WHY you would want to do this. Under normal circumstances, you probably wouldn't sit down and actually write all of this out. The idea here is that you can take a number and break it down into many different combinations of parts and still end up with the same total.

200 = 100 + 100
200 = 50 + 50 + 50 + 50
200 = 13 + 37 + 50 + 100

and so on...

You get the idea. It's just a basic concept.
• Great information, but my question would be is why would anyone would actually want to actually regroup whole numbers? or, is this going to be useful later in lessons somewhere?
• Money! If you owe someone \$25, do you have to pay them 2 tens and 5 ones? No, You could pay them 5 fives; or 25 ones; or 1 ten and 3 fives; or 1 ten 2 fives and 5 ones; etc.
• regrouing is important in math why?
• The concept is used as the basis for other things like borrowing and carrying.
• Can someone please clarify why I would ever want to regroup whole numbers? I do not think I have ever done so...
• Regrouping is needed in subtraction like 28-9=__ both the 8 and 9 is in the ones place but you cant take 9 from 8 so you regroup the 28 so the 2 would change to 1 and 8 into 18 so 18-9=9 and 10-00=10then just add to get the answer of 19
• How to round 234,489.0754 to the nearest thousand?
• Thousand or thousandths? There is a huge difference between the two.
Once you know which place value you want to round to, find that place in your number. Let's say you want to round to the nearest hundredth in your number. That's the second decimal place and it has a 7 in it. Find the number just to the right of that number. In this case, it's a 5 so we will round up. 234,489.0754 rounded to the nearest hundredth is 234,489.08.
Rounding works the same with or without decimals. Look at the number just to the right of the place value you want to round to. That number will tell you if you should round up or round down.
• when its like 86 and they say round to the nearest hundred how do I do that
• Let's say you want to round 123 to the nearest hundred. You look at the number to the right of the one you want to round to.
You want to round to the nearest hundred, so the place just to the right of that is the tens place. If that number is 5 or bigger, you round up. If it's 4 or smaller, you round down.
So with 123, the number in the tens place is a 2. That's 4 or smaller so you round down. 123 becomes 100.
Now let's look at 289 rounded to the nearest hundred. The number in the tens place is an 8. That's 5 or bigger so we round up. 289 becomes 300.

But what happens with numbers like 89?
Same thing. You want to round to the hundreds place so you look at the number in the tens place. What is it? 8. That's 5 or bigger so we round up. That means we have to increase the number that's in the hundreds place by 1. So what number is in the 100's place? 0. What's the next number up? 1. 89, rounded to the nearest hundred, is 100.