Codes: AC9M2N04, AC9M2A02. Goal: solve a two-digit subtraction with part-part-whole reasoning. Prepare: 42 counters in bundles, part circles, number line.
- Invite · 2 min. Ask: “42 counters were packed. 17 are used. How many are still packed? Could an addition fact help?”
- Model · 5 min. Show 42 as 4 tens and 2 ones. To remove 17, exchange one ten for ten ones; remove 7 ones and 1 ten. Three tens become two tens, five ones remain: 25. Check
25 + 17 = 42by making 10 from 5 + 7 and the tens. Name the exchange, not “borrowing.” - Guided missing part · 6 min. Place 42 in the whole circle, 17 in one part, blank in the other. Count from 17 to 20 (3), 20 to 40 (20), 40 to 42 (2); total jump 25. Match both models.
- Partner practice · 7 min. Solve 34 − 8 and 53 − 21. Offer objects or a number line. Partners check by adding the answer back: 26 + 8 = 34; 32 + 21 = 53. Ask which model they trust and why.
- Context · 3 min. “There are 53 library labels; 21 have been used. How many remain?” Learners say 32 labels, not just “32”.
- Check · 2 min. Everyone solves 31 − 6 using objects/line and writes or says the checking addition
25 + 6 = 31. Sample focus learners. If exchange is uncertain, return to 3 tens/1 one with actual pieces.
Access routes: tactile bundles, movable number line, oral/AAC part names, adult moves pieces at learner direction. Record strategy separately from notation.