🖐️ Abacus • vertical sums
bead simulation
Left hand (tens) · Right hand (ones) · +10 = +50−40 · stacked problems
✨ 0–99 · 6 numbers
✔️ bead‑accurate · vertical layout
📘 how bead‑accurate generation works
- Each rod (tens/ones) has upper=5, lower=1..4. Real bead transitions are simulated.
- Small friend: used when adding 1‑4 with insufficient lower beads (upper free).
- Big friend: adding 6‑9 with carry, or subtracting 6‑9 with borrow.
- Direct moves allowed in any problem. At least one step uses selected formula (if any).
- Problems are built step‑by‑step, rejecting any invalid transition.
- Now with vertical display – numbers stacked for easier abacus practice.