🔢 MathGrade 3Lesson 3
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Round Numbers to the Nearest 10 and 100

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Learning Objectives

  • 1Round a 2-digit number to the nearest 10 on a number line
  • 2Round a 3-digit number to the nearest 100 on a number line
  • 3Explain why a number rounds up or down based on the digit in the rounding place
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Mini Lesson

Rounding means finding the nearest 10 or 100 that a number is closest to. It helps us estimate quickly and work with simpler numbers. We use a number line to see exactly which "round" number a value is closest to.

The Rounding Rule

  • Look at the digit to the right of the place you are rounding to.
  • If that digit is less than 5, round down — keep the rounding-place digit the same.
  • If that digit is 5 or more, round up — increase the rounding-place digit by 1.
  • Replace all digits to the right of the rounding place with zeros.

Rounding to the Nearest 10

  • Look at the ones digit. If it is less than 5, round down: 23 → 20.
  • If the ones digit is 5 or more, round up: 47 → 50.
  • On a number line, 47 is between 40 and 50 and is closer to 50.

Rounding to the Nearest 100

  • Look at the tens digit. If it is less than 5, round down: 340 → 300.
  • If the tens digit is 5 or more, round up: 180 → 200.
  • A number exactly at the midpoint (e.g. 450) rounds up by convention.
Round 47 to the nearest 10A number line from 40 to 50 with 47 marked. An arrow shows 47 rounding up to 50 because the ones digit 7 is 5 or more.Round to the Nearest 104045504747 rounds UP to 50ones digit 7 ≥ 5 → round up
The Rounding RuleTwo panels showing the rounding rule. Left panel: ones digit less than 5 means round down, example 23 to 20. Right panel: ones digit 5 or more means round up, example 47 to 50.The Rounding Ruledigit < 5Round DOWN23 → 20digit ≥ 5Round UP47 → 5083 → 80 (3 < 5)65 → 70 (5 ≥ 5)Same rule works for hundreds too!
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To round to the nearest 10: look at the ones digit. To round to the nearest 100: look at the tens digit.

Vocabulary
Round
To change a number to the nearest 10, 100, or another place valuee.g. 47 rounds to 50.
Nearest
The closest numbere.g. The nearest 10 to 47 is 50.
Round up
To go up to the next 10 or 100 when the rounding digit is 5 or moree.g. 78 rounds up to 80.
Round down
To stay at the lower 10 or 100 when the rounding digit is less than 5e.g. 32 rounds down to 30.
Estimate
A close guess of an answer using rounded numberse.g. Round to estimate 47 + 32 as 50 + 30 = 80.
Midpoint
The number exactly halfway between two rounded valuese.g. 45 is the midpoint between 40 and 50.
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← Lesson 2: Understand Thousands, Hundreds, Tens, OnesLesson 4: Compare Large Numbers →