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Round to the nearest ten
Rounding 862 to 860 and 421 to 420 keeps both values very close to the originals.
862 − 421 ≈ 860 − 420 = 440
Worked example
The exact difference is 441, so the estimate 440 is only one away from the exact answer.
Nearest hundred also works for a rough check
For a rougher estimate, 862 ≈ 900 and 421 ≈ 400, giving 500. That is useful for magnitude checking, but rounding to tens is much more precise here.
Common mistakes
- Subtracting rounded values in the wrong order.
- Using too coarse a place value when a closer estimate is easy.
- Reporting the estimate as exact.
Frequently asked questions
What is the estimated difference of 862 and 421?
About 440 when rounded to the nearest ten.
What is the exact difference?
441.
Can I round to the nearest hundred?
Yes, but the estimate becomes rougher: 900 − 400 = 500.