Sum Detectives Lesson Plan Official Mission Dossier • Grade 2 Mathematics
Sum Detectives: The Great Regrouping Case
Unit: Place Value Addition • Pacing: 60 Minutes • Topic: 2-Digit Addition with Regrouping
Case Code #ADD-202
Learning Objectives (SWBAT)
Model 2-digit addition problems using base-ten blocks and place-value charts.Identify when regrouping is required (when the sum of the ones place is 10 or greater).Execute the exchange rule: bundle 10 ones into exactly 1 ten rod and place it in the tens column.Explain mathematical reasoning using precise vocabulary (addends, sum, regroup, place value).
Standards
CCSS.MATH.2.NBT.B.5: Fluently add within 100 using strategies based on place value.
CCSS.MATH.2.NBT.B.9: Explain why addition strategies work using place value models.
Detective Clue Words (Vocabulary)
Addend: A number added to another.
Sum: The total answer in addition.
Regroup: To rearrange 10 ones as 1 ten.
Base-Ten Rod: Value equal to 10 single units.
Detective Gear & Materials
Base-Ten blocks (rods and unit cubes: 6 tens, 20 ones per pair).
Laminated Place-Value Detective Mats & dry-erase markers.
Slide Presentation: Sum Detectives Case Brief .
Student Practice Worksheet & 3-minute Exit Ticket.
Common Investigation Pitfalls (Misconceptions)
Pitfall A: The "Double Squeeze" in Ones
Writing both digits in the ones place (e.g., \(28 + 15 = 313\) by writing 13 in ones). Intervention: Enforce the "No Room for Double Digits" rule with place-value room mats.
Pitfall B: The Phantom Ten
Bundling 10 ones but forgetting to add the new ten rod to the tens column. Intervention: Have students physically place a bright colored chip or marked rod on top of the tens attic.
Lesson Pacing:
10m: Hook & Case Intro • 15m: Direct Modeling • 15m: Guided Investigation • 12m: Independent Practice • 8m: Debrief & Exit
Sum Detectives • Lesson Plan Guide Page 1 of 2
Step-by-Step Investigation Protocol
Instructional delivery, teacher actions, student tasks, and differentiation.
Execution Guide
1 The Cold Case Hook: The Overflowing Ones Room (10 Minutes) Slide 1 & 2
Project the "Case of the Crowded Bank Vault" on the slides. Show 27 coins plus 15 coins. Invite students to count the single units: \(7 + 5 = 12\). Prompt: "Can 12 single dollar coins stay in the single-coin bin if the bin only fits up to 9 coins? What must we do?" Activate prior knowledge about making a group of 10.
2 Direct Modeling: The "10-for-1" Exchange Rule (15 Minutes) Slide 3 & 4
Demonstrate with oversized magnetic base-ten blocks on the board for \(38 + 25\):Step 1: Build 38 (3 tens, 8 ones) and 25 (2 tens, 5 ones) in the place-value columns.Step 2: Combine ones first: \(8 + 5 = 13\). Call out: "Ten-Alarm Alert! We have 10 or more!" Step 3: Count out exactly 10 ones, physically trade them for 1 ten rod, and move the new ten into the "Attic" of the tens column. Leave the remaining 3 ones.Step 4: Count total tens: \(1 + 3 + 2 = 6\). Final sum: \(63\).
3 Guided Practice: Partner Detective Duos (15 Minutes) Slide 5
Students work in partner pairs with manipulatives solving Case File #1 (\(47 + 26\)) and Case File #2 (\(35 + 19\)). Role A is the "Block Builder" (manipulating pieces), Role B is the "Lead Auditor" (checking if regrouping is needed and recording on the place-value chart). Switch roles between problems.
4 Independent Practice (12m)
Students solve the Sum Detectives Worksheet . Teacher circulates with a detective stamp, targeting students needing CRA (concrete-representational-abstract) scaffolding.
5 Debrief & Exit Ticket (8m)
Review the anchor sentence: "When ones reach ten, trade again!" Administer the 3-minute Exit Ticket to capture mastery data for tomorrow's groupings.
Differentiated Detective Tiers (MTSS / RTI)
Tier 1: Concrete Scaffold
Must use physical base-ten blocks and two-color ten-frame mats before recording any numerical symbols.
Tier 2: Representational
Draw quick-sketches (sticks for tens, dots for ones), circle 10 dots, and draw an arrow to the tens column.
Tier 3: Expert Extension
Missing addend mysteries (e.g., \(48 + \underline{\quad} = 73\)) and 3-addend detective challenges (\(24 + 18 + 16\)).
Language Support Sentence Frames
• "I have ones, which is (more than / less than) 9, so I (need to / do not need to) regroup."
Sum Detectives • Lesson Plan Guide Page 2 of 2
Sum Detectives Slides Math Detective Academy
Mission #ADD-202
SUM DETECTIVES
The Case of the Extra Ones: Mastering 2-Digit Addition with Regrouping
Goal: Learn when & how to bundle tens
Tool: Base-Ten Place Value Magic
Mystery Clue: The Crowded Vault! Case Brief #1
Only 9 single units can fit in the Ones Room!
Detective Sam collected 8 coins , then found 5 more coins .
8 + 5 = 13 Ones!
Warning: Alarm Triggered!
13 is too big for the Ones column!
What is our detective move when we have 10 or more ones?
Detective Question: Can two digits squeeze into one place-value column? Never!
The Golden Detective Rule The 10-for-1 Trade
10 Single Ones
10 single cubes
BUNDLE & TRADE! "Regroup"
1 Ten Rod
1 tens stick
"When ones hit ten, trade again!"
Case Study: 38 + 25 Step-by-Step
1
Add the Ones
8 ones + 5 ones = 13 ones.
13 is 10 + 3
2
Trade & Carry
Keep 3 ones . Send 1 new ten up to the Tens Attic!
+1 to the Tens column!
3
Add All Tens
1 ten + 3 tens + 2 tens = 6 tens.
Total Sum = 63!
Remember the Attic: Don't leave the new ten behind! 38 + 25 = 63
Detective Duo Challenge Solve With Your Partner
Case Clue on Board
47 + 26 = ?
Grab your place-value blocks and test your detective skills!
A
Partner A: Builder
Build 47 and 26. Count the single cubes.
B
Partner B: Auditor
Check if you need to trade 10 ones for 1 ten rod!
Quick Clue: 7 + 6 = 13. How many stay in ones? How many go to tens?
Case Closed: Detective Checklist Mastery Rules
Rule 1
Always start adding with the ones column first .
Rule 2
If ones are 10 or more , regroup 10 ones for 1 ten.
Rule 3
Always remember to add the regrouped ten in the tens total!
Sum Detectives Practice Worksheet Junior Math Detective Bureau • Field Case File
Sum Detectives: Case File #1
CASE: ADD-202
Detective Name:
Badge #:
Date:
Part 1: The Ten-Alarm Check • Does the ones column need regrouping?
Circle YES or NO
A. \(7 + 5 =\)
YES NO
B. \(4 + 3 =\)
YES NO
C. \(8 + 6 =\)
YES NO
D. \(9 + 1 =\)
YES NO
Part 2: Base-Ten Column Investigations
Add the ones first. If ones are 10 or more, write the regrouped ten in the dotted attic circle!
Case 1 36 + 27
Tens Ones
3 6
+2 7
6 + 7 = _____ ones → Regroup? [ ]
Case 2 54 + 28
Tens Ones
5 4
+2 8
4 + 8 = _____ ones → Regroup? [ ]
Case 3 49 + 35
Tens Ones
4 9
+3 5
9 + 5 = _____ ones → Regroup? [ ]
Detective Golden Clue: Always bundle 10 ones into 1 ten and carry it over!
Rule #1
Sum Detectives • Student Practice Worksheet Page 1 of 2
Field Cases: Evidence & Problem Solving
Solve the mystery equations and catch the error in the suspect's math!
Part 3 & 4
Part 3: Field Investigation Equations
Case 4
45
+38
Case 5
63
+29
Case 6
27
+56
Case 7
18
+74
Case File #8: The Footprint Mystery
Detective Maya found 38 muddy footprint clues outside the garden gate. Detective Leo found 25 more footprint clues by the old fountain. How many footprints did they find altogether?
Show Your Work (Diagram or Vertical Addition):
Equation:
Total Footprints:
Case File #9: Suspect Sid's Math Blunder!
Catch the Error
Sid's Bad Math
28
615
What mistake did Suspect Sid make?
Correct Sum: \(28 + 47 =\)
Sum Detectives • Student Practice Worksheet Page 2 of 2
Sum Detectives Exit Ticket Daily Mission Check • Quick Assessment
Sum Detectives: Case Closed Exit Ticket
3 Minutes
Detective Name:
Date:
1 Clue Check 1: Solve the Mystery Sum
Remember the Tens Attic!
Tens Ones
3 7
+4 5
Step-by-step thinking:
• \(7 + 5 =\) ones.
• Did you regroup?
• Total sum:
2
Clue Check 2: The Magnifying Glass Mystery
Detective Carlos needs 50 magnifying glasses for the academy. He found 24 in the cabinet and 28 in the safe . Does he have enough?
Workspace:
Total glasses found:
Does he have enough? (Circle one): YES NO
Because
3 Detective Rule Verification: "We only regroup when the sum of the ones is 10 or greater."
TRUE FALSE
Detective Self-Reflection • How do you feel about 2-digit regrouping?
🔍 Rookie Clue I still need help with the blocks.
🕵️♂️ Field Detective I can do it if I take my time.
⭐ Master Sleuth I can explain it to a partner!
Sum Detectives • Exit Ticket Formative Check Mission Complete
Sum Detectives Answer Key Official Solution Dossier • Teacher & Evaluator Copy
Sum Detectives: Master Answer Key
Complete solutions and step-by-step place-value annotations for Case File #ADD-202.
Confidential ANSWER KEY
Part 1: The Ten-Alarm Check Solutions
A. 7 + 5 = 12
YES (Regroup)
12 ≥ 10
B. 4 + 3 = 7
NO
7 < 10
C. 8 + 6 = 14
YES (Regroup)
14 ≥ 10
D. 9 + 1 = 10
YES (Regroup)
Exact 10
Part 2: Base-Ten Column Solutions
Case 1 36 + 27 = 63
TO
1
36
+ 27
63
6+7=13 • Trade 10 • Regroup: YES
Case 2 54 + 28 = 82
TO
1
54
+ 28
82
4+8=12 • Trade 10 • Regroup: YES
Case 3 49 + 35 = 84
TO
1
49
+ 35
84
9+5=14 • Trade 10 • Regroup: YES
Part 3: Vertical Algorithm Solutions
Case 4
Attic: [1]
45 + 38
= 83
(5+8=13; 1+4+3=8)
Case 5
Attic: [1]
63 + 29
= 92
(3+9=12; 1+6+2=9)
Case 6
Attic: [1]
27 + 56
= 83
(7+6=13; 1+2+5=8)
Case 7
Attic: [1]
18 + 74
= 92
(8+4=12; 1+1+7=9)
Sum Detectives • Teacher Answer Key Page 1 of 2
Investigation Solutions & Grading Guide
Explanations, Exit Ticket scoring, and diagnosis matrix.
Rubric & Diagnosis
Case File #8: Footprint Mystery Solution
Equation: 38 + 25 = 63
Work / Reasoning: Add ones: \(8 + 5 = 13\) ones. Regroup 13 ones into 1 ten and 3 ones. Add tens: \(1 \text{ (regrouped)} + 3 + 2 = 6\) tens. Total = 63 footprints .
Case File #9: Suspect Sid's Math Blunder Solution
Diagnosis of Error: Suspect Sid added the ones column \(8 + 7 = 15\) and recorded the full two-digit number 15 directly in the ones column instead of regrouping 10 ones as 1 ten into the tens column. Then he added \(2 + 4 = 6\), creating the incorrect number 615.
