📖 Today’s lesson
Lesson 21: Build infrastructure, not one-offs.
Every reusable formula, template, and named-range structure pays interest daily. The model writes the Excel machinery; you approve the logic.
Why this matters: Half of accounting runs on spreadsheets built at midnight years ago. The model writes clean AR-aging formulas, error-checks, and conditional flags in seconds — modernizing one workbook per week compounds fast.