MathK-2
Subtraction Detective: Finding Regrouping Errors
By Andrea F.
A practice set for 2nd graders where students play detective, examining Sam's three-digit subtraction work to locate regrouping mistakes, explain what went wrong, and solve each problem correctly. It builds place-value understanding and error analysis, with a three-step "Math Detective" checklist and written explanation prompts to guide thinking.
Customize for your classroom
Teacher Guide & Tips
- Use as a 30-40 minute independent or partner practice block after students have been introduced to subtraction with regrouping across the tens and hundreds.
- Discussion question: Why did Sam's answer for 452 - 128 come out too large, and what does "borrowing" from the tens place actually change?
- Scaffold for struggling readers: Model the first problem together with base-ten blocks, then have students use the three detective checks (ones, tens, zeros) as a written checklist before explaining.
- Extension for advanced students: Ask them to create their own "Sam's work" problem with a deliberate zero-regrouping error and trade with a partner to find it.
- Estimated time: about 5 minutes per error-analysis problem, plus a few minutes for the written explanation lines.
MathK-2
Subtraction Detective: Finding Regrouping Errors
By Andrea F.
A practice set for 2nd graders where students play detective, examining Sam's three-digit subtraction work to locate regrouping mistakes, explain what went wrong, and solve each problem correctly. It builds place-value understanding and error analysis, with a three-step "Math Detective" checklist and written explanation prompts to guide thinking.
Customize for your classroom
Teacher Guide & Tips
- Use as a 30-40 minute independent or partner practice block after students have been introduced to subtraction with regrouping across the tens and hundreds.
- Discussion question: Why did Sam's answer for 452 - 128 come out too large, and what does "borrowing" from the tens place actually change?
- Scaffold for struggling readers: Model the first problem together with base-ten blocks, then have students use the three detective checks (ones, tens, zeros) as a written checklist before explaining.
- Extension for advanced students: Ask them to create their own "Sam's work" problem with a deliberate zero-regrouping error and trade with a partner to find it.
- Estimated time: about 5 minutes per error-analysis problem, plus a few minutes for the written explanation lines.