🔢 MathGrade 3Lesson 7
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Estimate Sums and Differences
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Learning Objectives
- 1Round each addend or subtrahend to the nearest hundred before computing
- 2Use rounded numbers to estimate sums and differences of 3-digit numbers
- 3Explain whether an estimate is an overestimate or an underestimate
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Mini Lesson
Estimating means finding an answer that is close to the exact answer, but easier to calculate. To estimate sums and differences of 3-digit numbers, we round each number to the nearest hundred first, then add or subtract the rounded numbers.
Rounding to the Nearest Hundred
- Look at the tens digit. If it is 5 or more, round up. If it is less than 5, round down.
- 483 → tens digit is 8, which is 5 or more → round up to 500.
- 218 → tens digit is 1, which is less than 5 → round down to 200.
Estimating a Sum
- Round each addend to the nearest hundred.
- Add the rounded numbers: 500 + 200 = 700.
- Write: 483 + 218 ≈ 700 (the ≈ symbol means "approximately equal to").
Overestimates and Underestimates
- If you rounded UP, your estimate is larger than the exact answer — that is an overestimate.
- If you rounded DOWN, your estimate is smaller than the exact answer — that is an underestimate.
- Both are useful as long as you know which direction you rounded.
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Use the symbol ≈ to show your answer is an estimate, not exact.
Vocabulary
Estimate
A close guess of an answer using rounded numberse.g. I estimate 47 + 32 as 50 + 30 = 80.
Sum
The answer to an addition probleme.g. The sum of 12 + 8 is 20.
Difference
The answer to a subtraction probleme.g. The difference of 50 - 20 is 30.
Round
To change a number to the nearest 10, 100, or other place valuee.g. Round 47 to the nearest 10 to get 50.
Approximate (≈)
A symbol that means about equal toe.g. 47 + 32 ≈ 80.
Exact
The precise answer with no roundinge.g. The exact answer is 79; my estimate is 80.
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