Estimating a difference involves rounding the minuend and subtrahend independently to the nearest ten or hundred, then subtracting the rounded numbers to obtain a close, manageable value. For whole numbers through 1,000, a number line or place-value reasoning shows why the estimate may be greater or less than the exact difference and supports checking whether an answer is reasonable; decimals, larger numbers, and more general error analysis are beyond this scope.
To estimate a difference, round both numbers independently to the nearest ten or hundred. Then subtract the rounded numbers.
Example: Estimate by rounding to the nearest ten.
Round each number:
Subtract the rounded numbers:
So, the estimated difference is:
The exact difference is , so is a reasonable estimate. The estimate is a little less because was rounded down and was rounded up.
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