Solution to Challenge 1

Steps
1. Lay the A4 sheet portrait. Fold the bottom-left corner up so the bottom edge lies along the right-hand long edge. Crease firmly.
2. Remove the strip above the triangle (fold back and forth along the crease and tear, or cut).
3. Unfold the triangle.

Checkpoints
- Side length: 210 mm on every side (297 - 210 = 87 mm of strip is removed).
- Fold the square along one diagonal, then the other. The corners should meet exactly each time.
- Typical hand-folded result: diagonals agree to within 1-2 mm. Anything more means a side is off and the square should be trimmed.