Estimating by rounding
Estimate the answer to a calculation and use inverse operations to check answers; apply to increasingly large numbers using rounding and inverse reasoning
What a learner can do afterwards
- Round numbers to the nearest 10 or 100 to estimate a sum or difference before calculating
- Use addition to check a subtraction answer, or vice versa
- Identify when a calculated answer is unreasonable based on the estimate
- Round numbers to estimate a sum or difference before calculating
- Use addition to check a subtraction answer and vice versa
- Identify an unreasonable answer by comparing to the estimate
1 · Read
Before you work out an exact answer, you can make a fast guess called an estimate. Round each number to the nearest 10 or 100, then add or subtract the rounded numbers. This tells you about how big the real answer should be.
Mia adds 213 and 189. She rounds 213 to 210 and 189 to 190, so her estimate is 210 + 190 = 400. Then she works out the exact sum: 213 + 189 = 402. Since 402 is close to her estimate of 400, Mia knows her answer makes sense.
You can also check an exact answer with the opposite operation. If you added two numbers, subtract one of them from your answer and the other number should come back. If you subtracted, add your answer to the number you took away and you should land on the number you started with. An estimate tells you the answer is about right; the opposite operation tells you it is exactly right.
Round numbers to estimate first, then check your exact answer with the opposite operation and compare it to your estimate.
More to read
Add decimals: FAQ
How can estimating help when I add? Estimating means finding an answer that is close, but not exact. Round each number to a friendly number first, then add. For example, to add 12 dollars and 34 cents plus 5 dollars and 67 cents, round each amount to the nearest dollar. That gives 12 and 6. Now add: 12 + 6 = 18. The real answer is about 18 dollars and 1 cent, so the estimate was right on track. Estimating is a wonderful checker. After you find the exact answer, look back at your estimate. If the two are close, you can feel good about your work. If the exact answer is way off, like 81 dollars, you know a mistake is hiding somewhere. Hunt for it, fix it, and you are back on track.
2 · Watch
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Where it sits
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Where this leads
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