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Rounding

Rounding to the Nearest Ten


Imagine you have 29 marbles. If a friend asks how many you have, you could count every single one. But what if you wanted to give a quick answer? You might say, "I have about 30 marbles."
When you do this, you are rounding. We round numbers to make them simpler and easier to work with.

Definition Rounding
Rounding means changing a number to a simpler nearby number.
Method Rounding to the Nearest Ten
To round to the nearest ten, we only need to look at the ones digit.
  • If the ones digit is 4 or less (0, 1, 2, 3, 4), we round down. The tens digit stays the same, and the ones digit becomes 0.
  • If the ones digit is 5 or more (5, 6, 7, 8, 9), we round up. We add one to the tens digit, and the ones digit becomes 0.
Example
Round 324 to the nearest ten.

  1. Identify the ones digit: The number is \(32\underline{4}\). The ones digit is 4.
  2. Apply the rule: Since 4 is "4 or less," we round down. The tens digit (2) stays the same.
  3. Result: 324 rounded to the nearest ten is 320.
The number line shows that 324 is closer to 320 than it is to 330.

Rounding to the Nearest Hundred


Just as we can round to the nearest ten, we can also round to the nearest hundred for an even simpler number.
  • Instead of saying "A school has 372 students," you could say, "The school has about 400 students."
This is helpful when you don't need the exact number.

Method Rounding to the Nearest Hundred
The rule for rounding is the same! But to round to the nearest hundred, we look at the digit one place to the right: the tens digit.
  • If the tens digit is 4 or less (0, 1, 2, 3, 4), we round down. The hundreds digit stays the same, and the tens and ones digits become 0.
  • If the tens digit is 5 or more (5, 6, 7, 8, 9), we round up. We add one to the hundreds digit, and the tens and ones digits become 0.
Example
Round 324 to the nearest hundred.

  1. Identify the tens digit: The number is \(3\underline{2}\)4. The tens digit is 2.
  2. Apply the rule: Since 2 is "4 or less," we round down. The hundreds digit (3) stays the same.
  3. Result: 324 rounded to the nearest hundred is 300.
The number line shows that 324 is much closer to 300 than it is to 400.