1. Choose the source rule
2. Read result, unit and reach
Paper model only. A calculation cannot prove physical fit, safe mounting, material suitability, participation by every reader or a real event schedule. Verify local conditions separately with authorised adults.
3. Reader test
Can you find the source, unit and limit without guessing? Try zero, a whole count and an invalid entry. Sketch one clearer label or order, then retest with a consenting reader. A paper route gives the same calculations if a browser is unavailable.