Estimate Sums and Differences
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What you will learn
- 1Round each number 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
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
- When adding, if you rounded UP, your estimate is larger than the exact answer — that is an overestimate.
- When adding, if you rounded DOWN, your estimate is smaller than the exact answer — that is an underestimate.
- Both are useful. To be sure, compare your estimate to the exact answer: bigger is an overestimate, smaller is an underestimate.
Words to know
Estimate
A close guess of an answer using rounded numbers
e.g. I estimate 47 + 32 as 50 + 30 = 80.
Sum
The answer to an addition problem
e.g. The sum of 12 + 8 is 20.
Difference
The answer to a subtraction problem
e.g. The difference of 50 - 20 is 30.
Round
To change a number to the nearest 10, 100, or other place value
e.g. Round 47 to the nearest 10 to get 50.
Approximate (≈)
A symbol that means about equal to
e.g. 47 + 32 ≈ 80.
Exact
The precise answer with no rounding
e.g. The exact answer is 79; my estimate is 80.
Use the symbol ≈ to show your answer is an estimate, not exact.
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