Target: verify a three-set calculation by reconstructing exclusive regions and rejecting an impossible set claim. Prepare: C, region audit mat.
- Launch · 2 min. Ask whether a pairwise count of 4 in C means exactly 4 in A&R only. It includes the two all-three people.
- Model · 4 min. Centre 2; pair-only 2,1,3; single-only 7,4,3. Sum seven regions
2+2+1+3+7+4+3=22. Check A=7+2+1+2=12, R=4+2+3+2=11, C=3+1+3+2=9. - Guided reading · 5 min. Propose a claim “A&R pair=1 while all-three=2.” It is impossible because the triple is a subset of the pair. Discuss that nonnegative region counts and subset bounds are reasonableness tests.
- Practice route · 7 min. Choose Day 8 route; reconstruct and audit C's seven cells and reject one impossible alternative.
- Audit · 4 min. Repair any negative region by revisiting whether a pair count already includes all-three.
- Exit · 3 min. “What is the minimum possible size of each pair in C?” Expected: at least 2, the all-three count; actual pairs are 4,3,5.