Lesson 4.1 · Place Value and Rounding
Rounding to the nearest 10 and 100
Sometimes you don't need an exact number. If 47 kids come to a party, you might just say "about 50." Rounding turns a number into a nearby, friendlier number that is easy to think about.
Which ten is closer?
Every number sits between two tens. The number 47 is between 40 and 50. Look at where it lands on the number line:
47 is 3 steps away from 50 and 7 steps away from 40. It is closer to 50. So 47 rounded to the nearest ten is 50.
Definition
Rounding
Rounding means finding the nearby ten (or hundred) that a number is closest to.
The halfway rule
What about 45? It is exactly in the middle of 40 and 50. It is 5 steps from each one.
Mathematicians agreed on a rule for this: when a number is exactly halfway, round up. So 45 rounds to 50.
Rounding to the nearest ten
Look at the ones digit.
- If it is 0, 1, 2, 3 or 4, round down to the lower ten.
- If it is 5, 6, 7, 8 or 9, round up to the higher ten.
Worked example: Round to the nearest ten
Round 83 to the nearest ten.
83 is between 80 and 90. The ones digit is 3, which is less than 5. So 83 is closer to 80. It rounds down to 80.
The rule works for three-digit numbers too. Just find the two tens it sits between.
Worked example: A three-digit number
Round 276 to the nearest ten.
276 is between 270 and 280. The ones digit is 6, so round up. The answer is 280.
Rounding to the nearest hundred
To round to the nearest hundred, find the two hundreds a number sits between. The number 362 is between 300 and 400.
The halfway point is 350. 362 is past 350, so it is closer to 400. It rounds to 400.
This time, look at the tens digit. If it is 5 or more, round up. If it is 4 or less, round down.
Worked example: Round to the nearest hundred
Round 638 to the nearest hundred.
638 is between 600 and 700. The tens digit is 3, which is less than 5. So round down to 600.
Worked example: Right in the middle
Round 850 to the nearest hundred.
850 is exactly halfway between 800 and 900. The halfway rule says round up, so the answer is 900.
Common mistake
Look at the right digit! To round to the nearest ten, look at the ones digit. To round to the nearest hundred, look at the tens digit. For 348, the nearest ten is 350, but the nearest hundred is 300, because the tens digit is 4.
Tip
When you round, the digits to the right of the place you're rounding to always become 0. Rounding to the nearest hundred always gives an answer like 300, 700 or 1,000.
Practice
Round 67 to the nearest ten.
Type a number.
Round 43 to the nearest ten.
Type a number.
Round 85 to the nearest ten.
Type a number.
Round 462 to the nearest ten.
Type a number.
Round 348 to the nearest hundred.
Type a number.
Round 650 to the nearest hundred.
Type a number.
Which number rounds to 500 when you round to the nearest hundred?
A stadium has 996 seats. Round 996 to the nearest ten.
Type a number.