Addition errors with addends and sums within 1,000 can be identified by interpreting each digit according to its place value and checking whether ones, tens, and hundreds were combined and regrouped correctly. The reasoning distinguishes a computational mistake—such as failing to compose 10 ones or 10 tens—from a valid strategy written differently, and verifies the result through decomposition, an open number line, estimation, or related subtraction; formal algebraic error analysis is beyond this scope.
Look at each place value: ones, tens, and hundreds. Check whether a group of was regrouped into the next place.
Example:
This answer is not correct. Let’s check it from right to left.
Ones:
. Write in the ones place and regroup ones as ten.
Tens:
Add the regrouped ten:
. Write in the tens place and regroup tens as hundred.
Hundreds:
.
So the correct answer is:
The error was not adding the regrouped ten in the tens place. The answer is also too small: is close to , so makes sense.
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