Free public beta until December 4, 2026: every course is open, no card needed. After that, the first chapter of each course stays free and full access is a one-time $39 ($19 for up to 100 founding members chosen for helpful feedback). Your progress is kept. How the beta works

addition rule

P(A or B) equals P(A) plus P(B) minus their overlap.

For any two events: $P(A \cup B) = P(A) + P(B) - P(A \cap B)$ .