2. 1 400
250
643
390
60
700
10
When we round numbers we write them as
close to the original value as we can but with
less digits.
Whole numbers are usually rounded to the
nearest ten, hundred or thousand.
Rounded to the
nearest ten 640
643
Rounded to the
nearest hundred
600
643
Rounded to the
nearest thousand
1000
3. 1 400
250
643
390
60
700
10 A number line can help you to round numbers.
620 630 640 650 660 670 680 690
643 Rounded to the
nearest ten 640
Today we are going to look at how to round whole numbers
For example, lets start with the number 643. 643 can be rounded to the nearest ten. It would round to 640. It can also be rounded to the nearest hundred. It would round to or be written as 600. 643 could also be rounded to the nearest thousand. It would de rounded to or written as 1 000.
Why did we round or write the number 643 in the ways shown on the last slide?
Lets look at some number lines which show each of these roundings.
In this first number line, the line itself is divided into tens. Lets place 643 on the number line.
You can see that 643 is closer to the number 640 than it is to the number 650. That means if you were writing the number to the nearest ten, it would be rounded or written as 640 because that is a more accurate estimate than 650.
In this next number line, the line itself is divided into hundreds. Lets place 643 on the number line.
You can see that 643 is closer to 600 than it is to 700. That means if you were writing the number to the nearest hundred, it would be rounded or written as 600 because that is a more accurate estimate than 700.
When we say we have rounded 643 to the nearest hundred, we could also say that we are rounding it to the lead digit.
The lead digit is the first digit in a number. So in the case of 643, the lead digit is 6 or 600.
Let’s look at some more leading digits. In the number 6 000, the leading digit is the 6. It represents 6 thousand in place value.
The number 390 has 3 as its leading digit. The 3 has a place value of three hundreds.
Before we move on, let’s look at a couple more rounding problems using the number line. In this first one, the number line is again divided into tens between 6 420 and 6 490. This time, we are going to round 6 442 to the nearest ten.
6442 sits about here on the number line. You can see that it is much closer to 6 440 than it is to 6 450 . So, when we round 6442 to the nearest ten, we would write it as 6440.
In this example, the number line is divided into hundreds. This time, we are going to round 26 677 to the nearest hundred.
26 677 sits about here on the number line. You can see that it is much closer to 26 700 than it is to 26 600. So, when we round 26 677 to the nearest hundred, we would write it as 26 700.
Number lines are a great way to visualise rounding numbers. But you don’t have to use them all the time. A quicker way to round numbers is to follow a set of easy steps. Here is the number 583. Let’s round it to the nearest ten. The digit in the tens place is 8.
Let’s circle the number in the tens place. Because we are rounding the number to the nearest ten, we want to keep that and anything larger than it.
The digits we are going to keep are circled. Any smaller than that will be written as zeros. Now let’s look at the next number along, the three. The next number after the ones you are going to keep is the important one.
If that digit is 4 or smaller – that’s 4, 3, 2, 1 or zero – then the circled number next to it stays the same. The digit is three. So the circled digit, the eight stays the same and the three is replaced by a zero.
So 583 rounded to the nearest ten is 580
Now let’s look at what happens when the digit after the circled one is five or more. Here is the number 586. We will be rounding it to the nearest ten.
Let’s circle the number in the tens place. Remember, because we are rounding the number to the nearest ten, we want to keep that and anything larger than it.
Now let’s look at what happens when the digit next to the circled one is five or more. If that digit is 5 or bigger – that’s 5, 6, 7, 8, or 9 – then the circled number next to it will have one added to it. The digit is six. Because it is larger than 5, the circled number increases by one. In this case, the eight or eighty, becomes nine or ninety. And, as always, the numbers after the circled one become zeros.
So 586 rounded to the nearest ten is 590
In this next example, we are going to round 4 233 to the lead digit. The lead digit is the 4. It represents 4 000.
Circle the lead digit. That’s the only one you are going to keep as there are no digits larger than it.
Look at the digit next to the circled one. In this case it is the 2 or two hundred. That two is not five or more. That means when you re-write the number in its rounded form the circled number will stay the same. All the values after this are now written as zeros.
So 4 233 rounded to the lead digit is four thousand
To practice rounding whole numbers return to the OTLE page and try the activity.
