A Finite Projective Geometry Exercise 1.13.
One of the big misapprehensions about mathematics that we perpetrate in our classrooms is that the teacher always seems to know the answer to any problem that is discussed. This gives students the idea that there is a book somewhere with all the right answers to all of the interesting questions, and that teachers know those answers. And if one could get hold of the book, one would have everything settled. That's so unlike the true nature of mathematics.
—Leon Henkin, Teaching Teachers, Teaching Students (1981)

Exercise 1.13.  Write the dual for the axioms of a finite projective plane.

Dual of Axiom P1. For any two distinct lines, there is exactly one point incident with both lines.
Dual of Axiom P2. For any two distinct points, there is at least one line incident with both points.
Dual of Axiom P3. Every point has at least three lines incident with it.
Dual of Axiom P4. There exist at least four distinct lines of which no three are concurrent.