Lesson 10
All Sorts of Denominators
5.NF.A.1 · IM Grade 5, Unit 6, Lesson 10
Students add and subtract fractions where neither denominator divides the other. Two strategies: product of denominators, or finding a smaller common multiple.
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This lesson is a critical juncture for your class
MCT assessed 26 students with a diagnostic interview: an anchor task (1/2 + 1/5), a transfer task (2/3 + 1/4), and prerequisite probes when needed. Average interview: 7.2 minutes. 10 of 26 students will enter today with misconceptions that directly interfere with Activities 1 and 2.
Student Reasoning Map · Fraction Addition
L0
No partitioning
Marcus J.
L1
Halves only
Destiny W. Amira K.
L2
"Add everything"
Sofia M. Ethan C. Fatima A. Noah B. Lily T. Omar H.
L3
Procedure only
Isabella G. Liam P. Aisha N. Tyler K. Zara M. Jaylen
L4
Equivalence
Emma R. Carlos D. Maya S.
L5
Flexible
Jackson W. Chloe L. Leo F.
L6
Fluent
Olivia H. Nia J. Ryan T. Sophia C. Ben K.
Warm-Up How Many Do You See? Sofia's group will count squares but won't connect to fraction ops 10 to watch
Core Finding Common Denominators Highest risk. "Add everything" group will apply their broken algorithm 6 at risk
Students find values of 2/3 + 1/4 and 2/9 - 1/6 where neither denominator divides the other. IM monitors for multiples strategy vs. product strategy.
"Add everything" group will apply "add numerators, add denominators" and get 3/5 for 2/3 + 1/4. Consistent but wrong. Confident.
Sofia M. Ethan C. Fatima A. Noah B. Lily T. Omar H.
Procedure-only group will get correct answers but can't explain why common denominators work. All said "that's the rule."
Isabella G. Liam P. Aisha N. Tyler K. Zara M. Jaylen
MCT Adaptation
For the "add everything" group: hand them the warm-up area diagram alongside the expression. Ask them to write their answer first, then check against the diagram. The dissonance between "3/5" and "11/12" is the teaching moment. For the procedure-only group: ask Isabella or Jaylen to show on the area diagram where the twelfths come from.
If you hear
"I got 3/5" or "2+1 is 3 and 3+4 is 5"
Then ask
"Is 3/5 bigger or smaller than 2/3 by itself?"
If you hear
"That's just the rule" when asked about common denominators
Then say
Point to area diagram: "Show me where the 12 comes from."
Core Denominator Strategies Well-matched for most. Pair solid students with procedure-only students Most ready
Synthesis How Did You Decide? Listen for "that's the rule" answers. If you hear them, the gap persists Check understanding
Prep Checklist for Lesson 10
1
Bring fraction strips (halves, thirds, fourths, fifths, sixths). Place a set at Marcus, Destiny, Jaylen, and Amira's tables.
2
Seat Sofia, Ethan, Fatima, Noah, Lily, Omar near each other so you can circulate quickly during Activity 1.
3
Plan pairs for Activity 2: Jackson + Isabella, Emma + Liam, Carlos + Aisha, Maya + Jaylen.
4
Print the warm-up area diagram for the "add everything" group to reference during Activity 1.
After Lesson 10 · Quick Check
Answer these after class. MCT will use your responses to adjust Thursday's Lesson 11 prep.
Did the "add everything" group use a common denominator in the Cool-Down?
If no → MCT will flag a 10-minute small-group pull at the start of Lesson 11
Could any procedure-only student explain WHY common denominators work (not just how)?
If no → MCT will add a "same-size pieces" visual to Thursday's warm-up, targeting Isabella, Jaylen, and Liam
Did Marcus or Destiny engage with the fraction strips during the warm-up?
If no → MCT will recommend fraction strips all week + peer mentor for Lesson 11
The loop: Diagnose (MCT interviews) → Adapt (today's lesson) → Verify (this check) → Plan (tomorrow's Lesson 11). MCT updates Thursday's prep automatically.
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Wednesday, 7:15 AM
Ms. Thompson is prepping Lesson 10.
She has 26 students, a pacing guide, and 45 minutes.
MCT interviewed every one of them yesterday. It knows something about Jaylen, and 9 others, that will determine whether today's lesson lands or falls flat.
She doesn't need to change her plans. She needs to see what MCT sees.