Estimation of a difference involves replacing two-digit or accessible three-digit whole numbers up to 1,000 with nearby friendly tens or hundreds, then determining the approximate amount left by subtraction. Place value and number-line reasoning help judge whether the result should be close to zero, tens, or hundreds and check whether an exact difference is reasonable; formal error bounds, decimals, fractions, and estimation with larger numbers are beyond this understanding.
To estimate a difference, replace each number with a nearby friendly ten or hundred. Then subtract the friendly numbers.
Example: Estimate .
Find a nearby ten for each number:
Subtract the friendly numbers:
This makes sense because and are both close to and , and the amount left should be a few tens, not hundreds.
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