Sum Detectives Teacher Guide Teacher Resource 1.OA.4 • 10-Day Sequence
Sum Detectives Guide
Focus Strategy Missing Addends
Standard 1.OA.4 Target: Subtraction is taught as finding a missing partner in a part-part-whole relationship (e.g. solve 10 - 7 by thinking 7 + ? = 10). This prevents finger counting and establishes algebraic fluency.
Day(s) Strategy Focus Materials & Tools Formative Discussion Prompt 01 - 02 Ten-Frames Ten-Frame Mats Detective Worksheet "We have 6 red counters. How many yellow counters fill the rest of the frame to make 10?" 03 - 04 Bar Models Bar Model Mats Model Builders Sheet "If the Whole is 9 and one Part is 5, how many blocks live in Part B to make a train of 9?" 05 - 06 Number Lines Number Line Mats Line Hoppers Sheet "Point to the small number 4. How many hops forward does it take to reach the total of 8?" 07 - 08 Fact Families Fact Family Mats Family Houses Sheet "If 8, 2, and 10 live in the same house, how does 8 + 2 = 10 help us solve 10 - 8?" 09 - 10 Mixed Showdown Pathway Board Game Mixed Puzzle Worksheet "Which model helps you find the missing partner fastest? How did your teammate solve it?"
Common Clues (Pitfalls)
Finger Dependence: Remind students to find "partners" rather than "taking away."Addition Confusion: Students may add Whole + Part. Point out the "Total Attic" holds the biggest number.Hop Starting Error: Ensure they don't count "0" as the first hop on the number line.
Material Prep Checklist
Red/Yellow double-sided counters.
Unifix/Snap cubes in two colors.
Dry-erase pockets for lamination mats.
Print and cut scorecards & game pieces.
Remediation Strategy: Conduct 5-minute daily individual check-ins. If students struggle to associate subtraction with addition equivalents (1.OA.4), redirect them to the hands-on double-sided counter ten-frames before moving to abstract symbols.
Sum Detectives Sequence Guide — CCSS.MATH.1.OA.4 Unit Anchor
Page 1 of 1
Ten Frame Center Mats Center Activity 1 CASE: 1.OA.4.D1
Ten-Frame Detective Mat
Target Number
10
MISSION: Find the Missing Partner!
1. Read the case file. Place RED counters inside the ten-frame for the Known Part.
Case File #1: Solve 10 - 6 = ?
Clue: Known part is 6
R
R
R
R
R
R
Place
Place
Place
Place
How many yellow counters did you add?
6 + = 10
10 - 6 =
Case File #2: Solve 10 - 7 = ?
Clue: Known part is 7
R
R
R
R
R
R
R
Place
Place
Place
How many yellow counters did you add?
7 + = 10
10 - 7 =
Use dry-erase markers over laminated mats. Double-sided counters fit perfectly.
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Center Activity 2 CASE: 1.OA.4.D2
Undercover Totals Mat
Total Sums
Varies (7, 8, 9)
MISSION: Undercover Targets!
1. Check the target total in the magnifying glass.
Case File #3: Solve 8 - 5 = ?
Total: 8 Part: 5
R
R
R
R
R
Place
Place
Place
X
X
How many yellow counters did you add?
5 + = 8
8 - 5 =
Case File #4: Solve 7 - 4 = ?
Total: 7 Part: 4
R
R
R
R
Place
Place
Place
X
X
X
How many yellow counters did you add?
4 + = 7
7 - 4 =
Tip: Crossed out 'X' squares in the ten-frame mean those slots are inactive!
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Ten Frame Detective Worksheet Sum Detectives: Case File 1.1
Solve subtraction mysteries by drawing the missing partner!
Name:
Date:
CASE CLUE
Think Addition to Subtract!
Subtracting is just finding a missing partner. To solve 10 - 7 = ? , ask: 7 + ? = 10 . Draw empty circles in the empty spaces to find the secret partner!
CLUE FILE #01 Example
Mystery: 10 - 8 = 2
Addition Helper: 8 + 2 = 10
CLUE FILE #02 Your Turn
Mystery: 10 - 7 = ______
Addition Helper: 7 + ______ = 10
CLUE FILE #03 Your Turn
Mystery: 10 - 9 = ______
Addition Helper: 9 + ______ = 10
CLUE FILE #04 Your Turn
Mystery: 8 - 5 = ______
X
X
Addition Helper: 5 + ______ = 8
Detective Discussion Briefing:
Explain to your partner detective: How does drawing empty circles in the empty squares help you find the secret answer to subtraction? Write your thoughts:
CCSS.MATH.CONTENT.1.OA.A.4 - Understand subtraction as an unknown-addend problem
Page 1
Sum Detectives: Case File 1.2
Solve the puzzles by matching addition helpers to subtraction mysteries.
Name:
Date:
MISSION: Connect the Clues!
Draw a straight line to connect each Subtraction Mystery on the left to its matching Addition Helper on the right! Then, write the secret answer in both boxes.
1
10 - 4 = ____
5 + ____ = 9
A
2
9 - 5 = ____
4 + ____ = 10
B
3
7 - 2 = ____
2 + ____ = 7
C
4
10 - 5 = ____
5 + ____ = 10
D
Code-Cracking Challenge:
Mystery: 10 - 3 = . What addition sentence helps you find this missing partner?
+ =
CCSS.MATH.CONTENT.1.OA.A.4 - Understand subtraction as an unknown-addend problem
Page 2
Bar Model Center Mats Center Activity 3 CASE: 1.OA.4.D3
Bar Model Builder Mat
Strategy
Part-Part-Whole
MISSION: Snap the Cubes to Find the Part!
1. Build a train of cubes for the Known Part and place it on Part A.Missing Part and place it on Part B so the combined train matches the Total box!
Subtraction Case: 9 - 5 = ?
Clue: Total is 9, Known Part is 5
Total (Whole)
9
1
2
3
4
5
6
7
8
9
Known Part A
5
C
C
C
C
C
Missing Part B
?
Place Snap Cubes Here
Write down your evidence:
5 + = 9
9 - 5 =
Tip: Use 2 different colors of connecting blocks to show how they make 9.
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Center Activity 4 CASE: 1.OA.4.D4
Flexible Case Board
Target Level
Build-Your-Own
MISSION: Crack Your Own Cases!
1. Choose any subtraction equation (e.g., 8 - 3). Write the total at the top and the known part in Part A.
Custom Case File
Write & Wipe Board
Write the Total (Whole)
Write Known Part A
Place Cubes / Draw
Find Missing Part B
Place Cubes / Draw
Record Your Detective Work Below:
- =
Laminate this page to reuse with various dry-erase problems and counters.
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Bar Model Builders Worksheet Sum Detectives: Case File 2.1
Solve the bar models by finding the missing partner part!
Name:
Date:
CASE BRIEF
Bar Models show Parts and Wholes!
The Top Box is the Total (Whole). The Bottom Boxes are the Parts. If we know the Total and one Part, we subtract to find the other. Think: Known Part + ? = Total .
CASE RECORD #01 Solved Case
Total: 8
Part A: 5
Part B: 3
Addition: 5 + 3 = 8
Subtraction: 8 - 5 = 3
CASE RECORD #02 Your Turn
Total: 10
Part A: 6
Part B: ______
Addition: 6 + ______ = 10
Subtraction: 10 - 6 = ______
CASE RECORD #03 Your Turn
Total: 7
Part A: 4
Part B: ______
Addition: 4 + ______ = 7
Subtraction: 7 - 4 = ______
CASE RECORD #04 Your Turn
Total: 9
Part A: 7
Part B: ______
Addition: 7 + ______ = 9
Subtraction: 9 - 7 = ______
Detective Discussion Briefing:
Look at Case Record #03. Explain to another detective why the subtraction equation starts with the number 7, but the addition helper equation ends with the number 7:
CCSS.MATH.CONTENT.1.OA.A.4 - Understand subtraction as an unknown-addend problem
Page 1
Sum Detectives: Case File 2.2
Solve the puzzles by matching addition helpers to subtraction mysteries.
Name:
Date:
MISSION: Match the Bar Models!
Draw a straight line to connect each Bar Model Map on the left to its matching Equation Set on the right! Then, fill in the blanks in the equation cards.
CASE MAP #1
Total: 9
Part: 6
Part: ?
7 + ____ = 10
10 - 7 = ____
CODE: A
CASE MAP #2
Total: 8
Part: 4
Part: ?
6 + ____ = 9
9 - 6 = ____
CODE: B
CASE MAP #3
Total: 10
Part: 7
Part: ?
4 + ____ = 8
8 - 4 = ____
CODE: C
Code-Cracking Challenge:
A bar model has a Total of
Number Line Center Mats Center Activity 5 CASE: 1.OA.4.D5
Number Line Hopper Mat
Strategy
Hop-Forward Counting
MISSION: Hop Forward to Find the Secret Partner!
1. Start at the Known Part on the number line. Circle it with a dry-erase marker!Total . Circle the Total!
Subtraction Case: 10 - 7 = ?
Clue: Start at 7. Hop to 10.
Hopping Track
0
1
2
3
4
5
6
7
8
9
10
Write down your evidence:
7 + = 10
10 - 7 =
Use transparent counters as 'lilypads' to map out the hops visually.
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Center Activity 6 CASE: 1.OA.4.D6
Double Hop Board
Target Level
2 Practice Tracks
Track A Case: Solve 9 - 6 = ?
Clue: Start at 6, Land on 9
0
1
2
3
4
5
6
7
8
9
10
6 + = 9
9 - 6 =
Track B Case: Solve 8 - 4 = ?
Clue: Start at 4, Land on 8
0
1
2
3
4
5
6
7
8
9
10
4 + = 8
8 - 4 =
Laminate to write, wipe, and practice with other subtraction equations up to 10!
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Number Line Hoppers Worksheet Sum Detectives: Case File 3.1
Draw hopping arrows to solve the subtraction puzzles!
Name:
Date:
CASE BRIEF
Hop Forward from the Small Number!
To solve 10 - 8 = ? , start at 8 on the number line. Draw hops forward to land on 10 . Count how many hops you drew. That is the secret missing partner!
TRACKING RECORD #01 Example
Solve: 10 - 8 = 2
+1
+1
0
1
2
3
4
5
6
7
8
9
10
Addition Helper: 8 + 2 = 10 | Hops: 2
TRACKING RECORD #02 Your Turn
Solve: 10 - 6 = ______
0
1
2
3
4
5
6
7
8
9
10
Addition Helper: 6 + ______ = 10 | Hops: ______
TRACKING RECORD #03 Your Turn
Solve: 9 - 5 = ______
0
1
2
3
4
5
6
7
8
9
10
Addition Helper: 5 + ______ = 9 | Hops: ______
Detective Discussion Briefing:
Explain to a partner detective why we start hopping at the number 6 when we are solving 10 - 6. Why don't we start hopping at 0?
CCSS.MATH.CONTENT.1.OA.A.4 - Understand subtraction as an unknown-addend problem
Page 1
Sum Detectives: Case File 3.2
Solve the puzzles by matching addition helpers to subtraction mysteries.
Name:
Date:
MISSION: Match the Hop Maps!
Draw a straight line to connect each Number Line Map on the left to its matching Equation Box on the right! Then, fill in the blanks.
TRACK #1
0 2 4 6 8 10
5 + ____ = 10
10 - 5 = ____
CODE: A
TRACK #2
0 2 4 5 8 10
3 + ____ = 7
7 - 3 = ____
CODE: B
TRACK #3
0 2 3 5 7 10
4 + ____ = 8
8 - 4 = ____
Fact Family Houses Mats Center Activity 7 CASE: 1.OA.4.D7
Fact Family House Mat
Strategy
Related Equations
MISSION: Link the Family Numbers Together!
1. Look at the three family numbers in the attic roof (Total on top, Parts in the windows).
Fact Family: 10, 8, and ?
Clue: 10 - 8 = ? is in the same family as 8 + ? = 10
Total Attic
10
Part Window
8
Part Window
?
Family Equations Board (Trace & Write):
Addition Family 1
8 + = 10
Subtraction Family 1
10 - 8 =
Addition Family 2
Subtraction Family 2
10 - = 8
Tip: Use cubes on the windows to physically check the math relations.
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Center Activity 8 CASE: 1.OA.4.D8
Custom Neighborhood Mat
Target Level
Build-Your-Own
MISSION: Build Your Own Neighborhood!
1. Pick three numbers that belong to a fact family (e.g., 9, 5, 4).
House Case #1
____ + ____ = ____
____ + ____ = ____
____ - ____ = ____
____ - ____ = ____
House Case #2
____ + ____ = ____
____ + ____ = ____
____ - ____ = ____
____ - ____ = ____
Laminate this page to reuse with various dry-erase problems and counters.
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Fact Family Houses Worksheet Sum Detectives: Case File 4.1
Solve the fact family houses by finding the missing related numbers!
Name:
Date:
CASE BRIEF
Fact Families Live Together!
Three numbers live together in a fact family house: the Total lives in the roof attic, and the two Parts live in the window rooms. You can write 2 addition equations and 2 subtraction equations using these same three numbers!
HOUSE RECORD #01 Example
Total: 10
P: 7 P: 3
7 + 3 = 10 3 + 7 = 10
10 - 7 = 3 10 - 3 = 7
HOUSE RECORD #02 Your Turn
Total: 9
P: 5 P: ______
5 + ___ = 9 ___ + 5 = 9
9 - 5 = ___ 9 - ___ = 5
HOUSE RECORD #03 Your Turn
Total: 8
P: 6 P: ______
6 + ___ = 8 ___ + 6 = 8
8 - 6 = ___ 8 - ___ = 6
HOUSE RECORD #04 Your Turn
Total: 10
P: 4 P: ______
4 + ___ = 10 ___ + 4 = 10
10 - 4 = ___ 10 - ___ = 4
Detective Discussion Briefing:
Look at House Record #03. If you know that 6 + 2 = 8, what is the secret shortcut to find the answer to 8 - 6 without counting on your fingers?
CCSS.MATH.CONTENT.1.OA.A.4 - Understand subtraction as an unknown-addend problem
Page 1
Sum Detectives: Case File 4.2
Solve the puzzles by matching addition helpers to subtraction mysteries.
Name:
Date:
MISSION: Match the Family Houses!
Draw a straight line to connect each Family House Card on the left to its matching Equation Board on the right! Then, fill in the missing numbers.
HOUSE CARD 1
Roof: 9
P: 6 P: ?
7 + ____ = 10
10 - 7 = ____
CODE: A
HOUSE CARD 2
Roof: 10
P: 7 P: ?
6 + ____ = 9
9 - 6 = ____
CODE: B
HOUSE CARD 3
Roof: 8
P: 3 P: ?
3 + ____ = 8
8 - 3 = ____
CODE: C
Code-Cracking Challenge:
A fact family roof holds the number 10 . One window holds 9 . What number must live in the other window? Draw the house!
CCSS.MATH.CONTENT.1.OA.A.4 - Understand subtraction as an unknown-addend problem
Sum Detective Showdown Game Game Center CASE: 1.OA.4.GAME
Sum Detective Showdown
Players 2 - 4 Agents
HOW TO PLAY:
1. Place markers on the START block. Roll a die (or spin a spinner) to move forward.Addition Helper sentence to solve the subtraction mystery!SHIELD CASE SOLVED wins!
Start
CASE #1 10 - 8 = ___ Think: 8 + ? = 10
CASE #2 8 - 5 = ___ Think: 5 + ? = 8
CASE #3 9 - 7 = ___ Think: 7 + ? = 9
CASE #4 10 - 6 = ___ Think: 6 + ? = 10
CASE #5 7 - 4 = ___ Think: 4 + ? = 7
BONUS Hop Forward 2 Spaces!
CASE #6 9 - 6 = ___ Think: 6 + ? = 9
CASE #7 10 - 4 = ___ Think: 4 + ? = 10
CASE #8 8 - 3 = ___ Think: 3 + ? = 8
CASE #9 10 - 5 = ___ Think: 5 + ? = 10
Shield Case Solved!
Have your strategies handy! Use ten-frames or bar models to show your proof.
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Sum Detective Scorecard
Record the equations from the spaces you land on!
Agent Name:
Roll #
Subtraction Case Mystery
Addition Helper Code
01
- =
+ =
02
- =
+ =
03
- =
+ =
04
- =
+ =
05
- =
+ =
Game Debriefing:
Write down which of the four strategies (ten-frames, bar models, number lines, or fact family houses) was your absolute favorite to use to solve subtraction, and why!
CCSS.MATH.CONTENT.1.OA.A.4 - Understand subtraction as an unknown-addend problem
Page 2
Sum Detective Puzzle Worksheet Sum Detectives: Case File 5.1
Use your favorite strategies to solve the final subtraction mysteries!
Name:
Date:
FINAL REPORT
Choose Your Detective Weapon!
You are now a Master Sum Detective! You can use ten-frames, bar models, or number lines to find the missing partner and solve subtraction. Solve each mystery below!
CASE CLUE #01 - TEN FRAME Solve
Solve: 10 - 7 = ______
Draw circles in the empty blocks to find the missing partner!
Addition Helper: 7 + ______ = 10
CASE CLUE #02 - BAR MODEL Solve
Solve: 9 - 6 = ______
Write the missing partner in the blank Part B box!
Total: 9
Part A: 6
Part B: ?
Addition Helper: 6 + ______ = 9
CASE CLUE #03 - NUMBER LINE Solve
Solve: 8 - 4 = ______
Draw the hoops forward from 4 to 8 on the track!
0 2 4 6 8 10
Addition Helper: 4 + ______ = 8
CCSS.MATH.CONTENT.1.OA.A.4 - Understand subtraction as an unknown-addend problem
Page 1
Sum Detectives: Case File 5.2
Solve the final matching puzzles to close the folder!
Name:
Date:
MISSION: Connect Diverse Strategy Maps!
Draw a straight line to connect each Strategy Model on the left to its matching Addition-Subtraction Code Set on the right! Then, fill in the missing numbers.
MAP #1 - BAR MODEL
Total: 10
Part: 8
Part: ?
5 + ____ = 9
9 - 5 = ____
CODE: A
MAP #2 - NUMBER LINE
0 5 9 10
8 + ____ = 10
10 - 8 = ____
CODE: B
MAP #3 - TEN FRAME
5 + ____ = 10
10 - 5 = ____
CODE: C
Ultimate Agent Badge:
Write a short message to thank your helper classmate who helped you solve all the subtraction secrets during these 10 days!
CCSS.MATH.CONTENT.1.OA.A.4 - Understand subtraction as an unknown-addend problem
Page 2
Sum Detective Case Cards Sorting Center CASE: 1.OA.4.CARDS
Case File Clue Cards
Activity Print & Cut Cards
TEACHER PREP & GAMEPLAY:
1. Cut along the dotted scissor lines ✂ to make 6 **Case Cards** (Page 1) and 6 **Strategy Clues** (Page 2).
CASE CARD A
10 - 7 = ______
Mystery Missing Partner
Find: 7 + ______ = 10
CASE CARD B
9 - 5 = ______
Mystery Missing Partner
Find: 5 + ______ = 9
CASE CARD C
8 - 6 = ______
Mystery Missing Partner
Find: 6 + ______ = 8
CASE CARD D
10 - 4 = ______
Mystery Missing Partner
Find: 4 + ______ = 10
Have your strategies handy! Use dry-erase markers over laminated cards.
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Sorting Center CASE: 1.OA.4.CLUES
Strategy Clue Cards
Visual Clues Match & Crack
HOW TO SORT:
Each card below represents a **Visual Clue** for a subtraction mystery card from Page 1! Match the ten-frame, bar model, or number line to its corresponding subtraction math sentence card.
STRATEGY CARD #1 Ten-Frame
Clue: 7 are filled. How many to fill the rest of 10?
STRATEGY CARD #2 Bar Model
Total: 9
Part A: 5
Part B: ?
Clue: One part is 5, Total is 9. Find Part B!
STRATEGY CARD #3 Number Line
0 2 4 6 8 10
Clue: Start at 6. Hop forward until you hit 8!
STRATEGY CARD #4 Fact Family
Attic: 10
Window: 4 Window: ?
Clue: 10 lives in the roof. One room holds 4!
Tip: Scramble the cut cards and lay them face-down for a Memory Match game!
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