Math Detectives Teacher Guide Small Group Intervention • Grades 1–2
Math Detectives Teacher Guide
15–20 Min Station
Focus: Addition & Subtraction ≤ 20
Lesson Target & Objective
Students decode and solve one-step word problems (join, separate, and compare situations) within 20 using concrete representations, double ten-frames, and part-part-whole bar models before writing equations.
Target Standards CCSS 1.OA.A.1 CCSS 2.OA.A.1 MP1 (Sense Making), MP4 (Modeling)
Pacing Protocol (15–20 Minutes Total)
Phase 1 • 3 Mins
Mystery Warm-Up
Quick subitizing or ten-frame flash (Slides 1–2). Anchor the visual model for 10 + n.
Phase 2 • 5 Mins
Guided Modeling
Think-aloud with Slide 3 or 4. Identify the story action: are we joining, taking away, or comparing?
Phase 3 • 8 Mins
Clue Card Hunt
Pairs draw Task Cards, model with counters on recording sheet, and write equation evidence.
Phase 4 • 4 Mins
Case Closed Debrief
Students share one case: justify whether they used addition or subtraction and prove why.
The 3-Step Detective Protocol (Teach Students to Verbalize)
1 Hunt the Clues
Read twice. Circle given quantities. Underline the question. Identify the story action (did groups come together or separate?).
2 Map the Evidence
Fill in double ten-frames or part-part-whole bar. Decide: are we finding the whole or finding a missing part?
3 Crack the Code
Write the equation with label: 8 + 6 = 14 clues. Check if the answer makes reasonable sense.
Small Group Prep Checklist: • 1 set of laminated Operation Clue Task Cards (8 cards total) • Double-sided Case Closed Recording Mats • 20 two-color counters or dry-erase markers per student • Math Detectives Slide Deck displayed on small-group monitor or tablet.
Mystery Math Mission • Small Group Intervention Series Page 1 of 2
Instructional Scaffolding & Solutions
Questioning Prompts & Task Card Solutions
Teacher Reference
Common Misconceptions & Responsive Coaching
Observed Misconception Targeted Detective Coaching
Relies on isolated keywords (e.g., assumes "more" always means add). "Read the story without numbers first. Did our character get more items or lose items? Act it out with two hands."
Counts all counters starting at 1 instead of counting on or making 10. "You already have 8 in your first ten-frame. Put 8 in your head, then count on from 8: 9, 10, 11..."
Subtracts smaller number from larger regardless of story role. "Where is our total (the whole)? Build the whole first, then take away the part we know to reveal the secret part."
Small-Group Discussion Prompts
To Prompt Action Analysis:
"What happened in the middle of the story? Did the group grow bigger or smaller?"
To Connect Models to Equations:
"Where do you see the 10 in your model? How did making ten help you solve faster?"
To Prompt Justification:
"How can you prove to your partner that your answer is reasonable without recounting?"
To Check Comparison Problems:
"Who has more? How many extras do they have? Are we finding a total or a difference?"
Operation Clue Task Cards 1–8: Solution Key
Case #1 (Join) 8 + 7 = 15 15 magnifying glasses
Case #2 (Separate) 16 - 9 = 7 7 footprints left
Case #3 (Part-Part) 9 + 6 = 15 15 badges in all
Case #4 (Compare) 14 - 8 = 6 6 more keys
Case #5 (Join-Change) 11 + 6 = 17 17 secret files
Case #6 (Take From) 18 - 5 = 13 13 flashlights
Case #7 (Compare) 17 - 9 = 8 8 fewer stamps
Case #8 (Join Total) 7 + 9 = 16 16 decoder rings
Mystery Math Mission • Small Group Intervention Series Page 2 of 2
Mystery Math Practice Slides Math Detectives Academy
Small-Group Guided Practice
Target: Word Problems ≤ 20
Crack the Case with Addition & Subtraction
Look for the story action, build your ten-frame evidence, and solve the mystery equations!
Case File Mission Detective Guide • Slide 1
Warm-Up
Make 10 to Add Quickly!
How many clues in all?
First Ten-Frame: 8 Clues Second: 5 Clues
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+1
+1
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Think Like a Detective:
"8 needs 2 more to make a full 10."
"10 plus the 3 left over makes 13!"
8 + 5 = 13
Strategy: Make Ten • Decomposition Slide 2
Case 1
The Mystery Footprints (Join)
Action: Joining Groups
“Detective Maya spotted 9 muddy footprints by the garden gate. Then she discovered 6 more footprints on the porch.”
Question: How many footprints did Maya find in all?
1. Story Clue Found MORE
2. Operation Add (+)
3. Solution 9 + 6 = 15
Check: 9 footprints + 1 footprint = 10, plus 5 = 15 footprints Slide 3
Case 2
The Locked Lockers (Separate)
Action: Taking Away
“Officer Leo gathered 15 locked clue boxes at the station. He used master keys to unlock 7 of the boxes.”
Question: How many boxes are STILL locked?
1. Story Clue Took Away / Left
2. Operation Subtract (−)
3. Solution 15 − 7 = 8
Check with Addition: 8 locked + 7 unlocked = 15 total boxes Slide 4
Case 3
Comparing Clue Tokens
Action: Comparing Two Groups
“Agent Ben earned 14 clue badges. Agent Sam earned 8 clue badges.”
Question: How many MORE badges did Ben earn than Sam?
Ben: 14 badges
14 Badges Total
Sam: 8 badges Difference: ?
8
Difference = 6
14 − 8 = 6 badges
Compare Rule: Larger Group − Smaller Group = Difference Slide 5
Station
Your Detective Partner Mission
Time to Investigate!
1
Pick a Clue Card
Operation Clue Task Cards Operation Clue Task Cards (Set A: Cases 1–4)
Cut along dashed lines • Station Cards
CASE #1 The Missing Magnifiers
Joining
Detective Noah gathered 8 magnifying glasses on the desk. Agent Zoe brought in 7 more glasses from the lab.
How many magnifying glasses do Noah and Zoe have in all?
Detective Clue: Are you putting groups together?
Record your model & equation on Case Mat #1
CASE #2 The Muddy Footprints
Separating
Officer Chloe tracked 16 muddy footprints on the sidewalk. Rain poured down and washed away 9 of the prints.
How many footprints can Chloe still see?
Detective Clue: Are you taking away from the total?
Record your model & equation on Case Mat #2
CASE #3 The Secret Badges
Part-Part
In the locked evidence safe, there are 9 gold detective badges and 6 silver detective badges.
How many detective badges are inside the safe in all?
Detective Clue: Combine part 1 and part 2 to find whole.
Record your model & equation on Case Mat #3
CASE #4 The Vault Keys
Compare
Detective Leo has 14 master keys on his ring. Detective Mia has 8 master keys on her ring.
How many more master keys does Leo have than Mia?
Detective Clue: Find the difference between two groups!
Record your model & equation on Case Mat #4
Mystery Math Mission • Small Group Activity Cards Page 1 of 2
Operation Clue Task Cards (Set B: Cases 5–8)
Cut along dashed lines • Station Cards
CASE #5 The Secret Files
Joining
Agent Sam solved 11 secret files on Monday. On Tuesday, he solved 6 more files.
How many secret files did Agent Sam solve altogether?
Detective Clue: Start with 11 and count on 6 more!
Record your model & equation on Case Mat #5
CASE #6 The Night Flashlights
Separating
The detective supply room had 18 flashlights ready to use. Night officers took 5 flashlights on patrol.
How many flashlights are left in the supply room?
Detective Clue: Start with 18. Take away the ones taken.
Record your model & equation on Case Mat #6
CASE #7 The Evidence Stamps
Compare
Officer Jordan stamped 17 case folders. Officer Tara stamped 9 case folders.
How many fewer case folders did Tara stamp than Jordan?
Detective Clue: Compare the two amounts to find difference.
Case Closed Student Worksheet CASE CLOSED: EVIDENCE RECORDING MAT
Small-Group Word Problem Station (Problems 1–4)
Name:
Date:
Detective Routine: 1. Check the story action • 2. Model in ten-frames • 3. Fill number bond • 4. Write number sentence
CASE #1 The Missing Magnifiers
Action: Join (+) Separate (−) Compare
Double Ten-Frame Evidence:
Number Bond
Equation:
+ / −
=
Answer:
CASE #2 The Muddy Footprints
Action: Join (+) Separate (−) Compare
Double Ten-Frame Evidence:
Number Bond
Equation:
+ / −
=
Answer:
CASE #3 The Secret Badges
Action: Join (+) Separate (−) Compare
Double Ten-Frame Evidence:
Number Bond
Equation:
+ / −
=
Answer:
CASE #4 The Vault Keys
Action: Join (+) Separate (−) Compare
Double Ten-Frame Evidence:
Number Bond
Equation:
+ / −
=
Answer:
Case Closed Recording Sheet • Mystery Math Mission Page 1 of 2
CASE CLOSED: EVIDENCE RECORDING MAT
Small-Group Word Problem Station (Problems 5–8)
Cases 5–8
CASE #5 The Secret Files
Action: Join (+) Separate (−) Compare
Double Ten-Frame Evidence:
Bond
Equation:
+ / −
=
Answer:
CASE #6 The Night Flashlights
Action: Join (+) Separate (−) Compare
Double Ten-Frame Evidence:
Bond
Equation:
+ / −
=
Answer:
CASE #7 The Evidence Stamps
Action: Join (+) Separate (−) Compare
Double Ten-Frame Evidence:
Bond
Equation:
+ / −
=
Answer:
CASE #8 The Decoder Rings
Action: Join (+) Separate (−) Compare
Double Ten-Frame Evidence:
Bond
Equation:
+ / −
=
Answer:
Detective Self-Check & Mission Reflection Check all that apply
How did I prove my answers?
