1. Fill tens
Tens window: 8 + 4 = 12. Hold raw 12; carry settles later.
3 visual steps / answer-window addition
Tens window: 8 + 4 = 12. Hold raw 12; carry settles later.
Ones window: 6 + 7 = 13. Hold raw 13; settle carry later.
Settle raw 12 | 13 into 133; keep ones and move carry left.
A few quick rounds make digit release automatic; carry is the only thing to keep.
Start Level